diff --git a/packages/auth/src/Authentication/AuthenticatorInitializer.php b/packages/auth/src/Authentication/AuthenticatorInitializer.php
index e87555096b..92a86d549c 100644
--- a/packages/auth/src/Authentication/AuthenticatorInitializer.php
+++ b/packages/auth/src/Authentication/AuthenticatorInitializer.php
@@ -8,7 +8,7 @@
 use Tempest\Container\Initializer;
 use Tempest\Container\Singleton;
 use Tempest\Http\Session\Session;
-use Tempest\Http\Session\SessionManager;
+use Tempest\Http\Session\SessionRegenerator;
 
 final readonly class AuthenticatorInitializer implements Initializer
 {
@@ -16,9 +16,9 @@
     public function initialize(Container $container): Authenticator
     {
         return new SessionAuthenticator(
-            sessionManager: $container->get(SessionManager::class),
             session: $container->get(Session::class),
             authenticatableResolver: $container->get(AuthenticatableResolver::class),
+            sessionRegenerator: $container->get(SessionRegenerator::class),
         );
     }
 }
diff --git a/packages/auth/src/Authentication/SessionAuthenticator.php b/packages/auth/src/Authentication/SessionAuthenticator.php
index 572b4fcfef..cc09a6cc39 100644
--- a/packages/auth/src/Authentication/SessionAuthenticator.php
+++ b/packages/auth/src/Authentication/SessionAuthenticator.php
@@ -5,7 +5,7 @@
 namespace Tempest\Auth\Authentication;
 
 use Tempest\Http\Session\Session;
-use Tempest\Http\Session\SessionManager;
+use Tempest\Http\Session\SessionRegenerator;
 
 final class SessionAuthenticator implements Authenticator
 {
@@ -20,9 +20,9 @@ final class SessionAuthenticator implements Authenticator
     private ?Authenticatable $current = null;
 
     public function __construct(
-        private readonly SessionManager $sessionManager,
         private readonly Session $session,
         private readonly AuthenticatableResolver $authenticatableResolver,
+        private readonly SessionRegenerator $sessionRegenerator,
     ) {}
 
     public function authenticate(Authenticatable $authenticatable): void
@@ -43,15 +43,19 @@ public function authenticate(Authenticatable $authenticatable): void
         $this->currentId = $id;
         $this->currentClass = $class;
         $this->current = $authenticatable;
+
+        // The session identifier must not survive a change in privilege level, or one
+        // known to an attacker before authentication stays valid afterwards.
+        $this->sessionRegenerator->regenerate();
     }
 
     public function deauthenticate(): void
     {
-        $this->session->remove(self::AUTHENTICATABLE_KEY);
-        $this->session->remove(self::AUTHENTICATABLE_CLASS);
         $this->clearCurrent();
 
-        $this->sessionManager->save($this->session);
+        // Regenerate session without preserving data to prevent session fixation
+        // and purge all authenticated user data.
+        $this->sessionRegenerator->invalidate();
     }
 
     public function current(): ?Authenticatable
diff --git a/packages/auth/tests/SessionAuthenticatorTest.php b/packages/auth/tests/SessionAuthenticatorTest.php
index 6e791e796d..45dd4e459a 100644
--- a/packages/auth/tests/SessionAuthenticatorTest.php
+++ b/packages/auth/tests/SessionAuthenticatorTest.php
@@ -13,7 +13,9 @@
 use Tempest\DateTime\DateTime;
 use Tempest\Http\Session\Session;
 use Tempest\Http\Session\SessionId;
+use Tempest\Http\Session\SessionIdResolver;
 use Tempest\Http\Session\SessionManager;
+use Tempest\Http\Session\SessionRegenerator;
 
 final class SessionAuthenticatorTest extends TestCase
 {
@@ -27,9 +29,9 @@ public function current_memoizes_the_resolved_authenticatable_for_the_current_se
         $session->set(SessionAuthenticator::AUTHENTICATABLE_CLASS, MemoizedAuthenticatable::class);
 
         $authenticator = new SessionAuthenticator(
-            sessionManager: new TestingSessionManager(),
             session: $session,
             authenticatableResolver: $resolver,
+            sessionRegenerator: $this->createRegenerator($session),
         );
 
         $this->assertSame($authenticatable, $authenticator->current());
@@ -46,9 +48,9 @@ public function current_memoizes_a_missing_authenticatable_for_the_current_sessi
         $session->set(SessionAuthenticator::AUTHENTICATABLE_CLASS, MemoizedAuthenticatable::class);
 
         $authenticator = new SessionAuthenticator(
-            sessionManager: new TestingSessionManager(),
             session: $session,
             authenticatableResolver: $resolver,
+            sessionRegenerator: $this->createRegenerator($session),
         );
 
         $this->assertNull($authenticator->current());
@@ -68,9 +70,9 @@ public function current_re_resolves_when_the_session_identity_changes(): void
         $session->set(SessionAuthenticator::AUTHENTICATABLE_CLASS, MemoizedAuthenticatable::class);
 
         $authenticator = new SessionAuthenticator(
-            sessionManager: new TestingSessionManager(),
             session: $session,
             authenticatableResolver: $resolver,
+            sessionRegenerator: $this->createRegenerator($session),
         );
 
         $current = $authenticator->current();
@@ -95,9 +97,9 @@ public function reset_clears_the_cached_current_authenticatable(): void
         $session->set(SessionAuthenticator::AUTHENTICATABLE_CLASS, MemoizedAuthenticatable::class);
 
         $authenticator = new SessionAuthenticator(
-            sessionManager: new TestingSessionManager(),
             session: $session,
             authenticatableResolver: $resolver,
+            sessionRegenerator: $this->createRegenerator($session),
         );
 
         $this->assertSame($authenticatable, $authenticator->current());
@@ -120,9 +122,9 @@ public function authenticate_replaces_a_cached_current_authenticatable(): void
         $session->set(SessionAuthenticator::AUTHENTICATABLE_CLASS, MemoizedAuthenticatable::class);
 
         $authenticator = new SessionAuthenticator(
-            sessionManager: new TestingSessionManager(),
             session: $session,
             authenticatableResolver: $resolver,
+            sessionRegenerator: $this->createRegenerator($session),
         );
 
         $current = $authenticator->current();
@@ -136,6 +138,58 @@ public function authenticate_replaces_a_cached_current_authenticatable(): void
         $this->assertSame(2, $current->id);
     }
 
+    #[Test]
+    public function authenticate_regenerates_the_session_identifier(): void
+    {
+        $session = $this->createSession();
+        $sessionManager = new TestingSessionManager();
+
+        $authenticator = new SessionAuthenticator(
+            session: $session,
+            authenticatableResolver: new CountingAuthenticatableResolver(),
+            sessionRegenerator: $this->createRegenerator($session, $sessionManager),
+        );
+
+        $authenticator->authenticate(new MemoizedAuthenticatable(id: 1));
+
+        $this->assertNotSame('test-session', (string) $session->id);
+        $this->assertSame(1, $sessionManager->deletedSessions);
+        $this->assertSame(1, $session->get(SessionAuthenticator::AUTHENTICATABLE_KEY));
+    }
+
+    #[Test]
+    public function deauthenticate_regenerates_the_session_identifier_and_discards_the_data(): void
+    {
+        $session = $this->createSession();
+        $session->set(SessionAuthenticator::AUTHENTICATABLE_KEY, 1);
+        $session->set(SessionAuthenticator::AUTHENTICATABLE_CLASS, MemoizedAuthenticatable::class);
+        $session->set('key', 'value');
+        $sessionManager = new TestingSessionManager();
+
+        $authenticator = new SessionAuthenticator(
+            session: $session,
+            authenticatableResolver: new CountingAuthenticatableResolver(),
+            sessionRegenerator: $this->createRegenerator($session, $sessionManager),
+        );
+
+        $authenticator->deauthenticate();
+
+        $this->assertNotSame('test-session', (string) $session->id);
+        $this->assertSame(1, $sessionManager->deletedSessions);
+        $this->assertNull($session->get(SessionAuthenticator::AUTHENTICATABLE_KEY));
+        $this->assertNull($session->get(SessionAuthenticator::AUTHENTICATABLE_CLASS));
+        $this->assertNull($session->get('key'));
+    }
+
+    private function createRegenerator(Session $session, ?SessionManager $sessionManager = null): SessionRegenerator
+    {
+        return new SessionRegenerator(
+            sessionManager: $sessionManager ?? new TestingSessionManager(),
+            session: $session,
+            sessionIdResolver: new TestingSessionIdResolver(),
+        );
+    }
+
     private function createSession(): Session
     {
         $now = DateTime::now();
@@ -186,10 +240,25 @@ public function resolveId(Authenticatable $authenticatable): int
     }
 }
 
+final class TestingSessionIdResolver implements SessionIdResolver
+{
+    public function resolve(): SessionId
+    {
+        return new SessionId('test-session');
+    }
+
+    public function issueNewId(): SessionId
+    {
+        return new SessionId('regenerated-session-' . uniqid());
+    }
+}
+
 final class TestingSessionManager implements SessionManager
 {
     public int $savedSessions = 0;
 
+    public int $deletedSessions = 0;
+
     public function getOrCreate(SessionId $id): Session
     {
         $now = DateTime::now();
@@ -202,7 +271,10 @@ public function save(Session $session): void
         $this->savedSessions++;
     }
 
-    public function delete(Session $session): void {}
+    public function delete(Session $session): void
+    {
+        $this->deletedSessions++;
+    }
 
     public function isValid(Session $session): bool
     {
diff --git a/packages/http/src/Session/Resolvers/CookieSessionIdResolver.php b/packages/http/src/Session/Resolvers/CookieSessionIdResolver.php
index 4c5077ac76..25b8831ff8 100644
--- a/packages/http/src/Session/Resolvers/CookieSessionIdResolver.php
+++ b/packages/http/src/Session/Resolvers/CookieSessionIdResolver.php
@@ -30,27 +30,37 @@ public function __construct(
 
     public function resolve(): SessionId
     {
-        $sessionKey = str($this->appConfig->name ?? 'tempest')
-            ->snake()
-            ->append('_session_id')
-            ->toString();
-
-        $id = $this->request->getCookie($sessionKey)?->value;
+        $id = $this->request->getCookie($this->getSessionKey())?->value;
 
         if (! $id) {
-            $id = (string) Uuid::v4();
-
-            $this->cookies->add(new Cookie(
-                key: $sessionKey,
-                value: $id,
-                expiresAt: $this->clock->now()->plus($this->sessionConfig->expiration),
-                path: '/',
-                secure: Str\starts_with($this->appConfig->baseUri, needles: 'https'),
-                httpOnly: true,
-                sameSite: SameSite::LAX,
-            ));
+            return $this->issueNewId();
         }
 
         return new SessionId($id);
     }
+
+    public function issueNewId(): SessionId
+    {
+        $id = (string) Uuid::v4();
+
+        $this->cookies->add(new Cookie(
+            key: $this->getSessionKey(),
+            value: $id,
+            expiresAt: $this->clock->now()->plus($this->sessionConfig->expiration),
+            path: '/',
+            secure: Str\starts_with($this->appConfig->baseUri, needles: 'https'),
+            httpOnly: true,
+            sameSite: SameSite::LAX,
+        ));
+
+        return new SessionId($id);
+    }
+
+    private function getSessionKey(): string
+    {
+        return str($this->appConfig->name ?? 'tempest')
+            ->snake()
+            ->append('_session_id')
+            ->toString();
+    }
 }
diff --git a/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php b/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php
index ecc26b7245..765c3513c9 100644
--- a/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php
+++ b/packages/http/src/Session/Resolvers/HeaderSessionIdResolver.php
@@ -30,4 +30,9 @@ public function resolve(): SessionId
             id: $this->request->headers[$sessionKey] ?? Uuid::v4()->toString(),
         );
     }
+
+    public function issueNewId(): SessionId
+    {
+        return new SessionId(id: Uuid::v4()->toString());
+    }
 }
diff --git a/packages/http/src/Session/Session.php b/packages/http/src/Session/Session.php
index 78846c4a92..677485a66f 100644
--- a/packages/http/src/Session/Session.php
+++ b/packages/http/src/Session/Session.php
@@ -132,6 +132,15 @@ public function cleanup(): void
         }
     }
 
+    /**
+     * @internal Prefer {@see SessionRegenerator}, which also destroys the session that is being
+     * replaced and sends the new identifier to the client.
+     */
+    public function replaceId(SessionId $id): void
+    {
+        $this->id = $id;
+    }
+
     /**
      * Clears all values from the session.
      */
diff --git a/packages/http/src/Session/SessionIdResolver.php b/packages/http/src/Session/SessionIdResolver.php
index 6b49d69ddb..aa52293593 100644
--- a/packages/http/src/Session/SessionIdResolver.php
+++ b/packages/http/src/Session/SessionIdResolver.php
@@ -6,5 +6,15 @@
 
 interface SessionIdResolver
 {
+    /**
+     * Resolves the identifier sent by the client, creating a new one if there is none.
+     */
     public function resolve(): SessionId;
+
+    /**
+     * Creates a new identifier and sends it to the client, replacing the one it was using.
+     *
+     * @see SessionRegenerator
+     */
+    public function issueNewId(): SessionId;
 }
diff --git a/packages/http/src/Session/SessionRegenerator.php b/packages/http/src/Session/SessionRegenerator.php
new file mode 100644
index 0000000000..9ee2afa123
--- /dev/null
+++ b/packages/http/src/Session/SessionRegenerator.php
@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tempest\Http\Session;
+
+/**
+ * Regenerates the session identifier to prevent session fixation attacks.
+ *
+ * @see https://cheatsheetseries.owasp.org/cheatsheets/Session_Management_Cheat_Sheet.html
+ * @see https://owasp.org/www-community/attacks/Session_fixation
+ */
+final readonly class SessionRegenerator
+{
+    public function __construct(
+        private SessionManager $sessionManager,
+        private Session $session,
+        private SessionIdResolver $sessionIdResolver,
+    ) {}
+
+    /**
+     * Assigns a new ID to the current session, carrying over data.
+     */
+    public function regenerate(): void
+    {
+        $this->sessionManager->delete($this->session);
+
+        $this->session->replaceId($this->sessionIdResolver->issueNewId());
+
+        $this->sessionManager->save($this->session);
+    }
+
+    /**
+     * Assigns a new ID to the current session, discarding all data.
+     */
+    public function invalidate(): void
+    {
+        $this->sessionManager->delete($this->session);
+
+        $this->session->replaceId($this->sessionIdResolver->issueNewId());
+        $this->session->clear();
+
+        $this->sessionManager->save($this->session);
+    }
+}
diff --git a/tests/Integration/Http/SessionRegeneratorTest.php b/tests/Integration/Http/SessionRegeneratorTest.php
new file mode 100644
index 0000000000..9518b6e099
--- /dev/null
+++ b/tests/Integration/Http/SessionRegeneratorTest.php
@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Tempest\Integration\Http;
+
+use PHPUnit\Framework\Attributes\Test;
+use Tempest\Http\Cookie\CookieManager;
+use Tempest\Http\Session\Session;
+use Tempest\Http\Session\SessionManager;
+use Tempest\Http\Session\SessionRegenerator;
+use Tests\Tempest\Integration\FrameworkIntegrationTestCase;
+
+/**
+ * @internal
+ */
+final class SessionRegeneratorTest extends FrameworkIntegrationTestCase
+{
+    private Session $session {
+        get => $this->container->get(Session::class);
+    }
+
+    private SessionRegenerator $regenerator {
+        get => $this->container->get(SessionRegenerator::class);
+    }
+
+    #[Test]
+    public function assigns_a_new_identifier_and_keeps_data(): void
+    {
+        $this->session->set('key', 'value');
+        $previousId = (string) $this->session->id;
+
+        $this->regenerator->regenerate();
+
+        $this->assertNotSame($previousId, (string) $this->session->id);
+        $this->assertSame('value', $this->session->get('key'));
+    }
+
+    #[Test]
+    public function invalidate_discards_data(): void
+    {
+        $this->session->set('key', 'value');
+
+        $this->regenerator->invalidate();
+
+        $this->assertNull($this->session->get('key'));
+    }
+
+    #[Test]
+    public function destroys_the_session_it_replaces(): void
+    {
+        $sessionManager = $this->container->get(SessionManager::class);
+
+        $this->session->set('key', 'value');
+        $previousId = $this->session->id;
+
+        $this->regenerator->regenerate();
+
+        $previousSession = $sessionManager->getOrCreate($previousId);
+
+        $this->assertNull($previousSession->get('key'));
+    }
+
+    #[Test]
+    public function sends_the_new_identifier_to_the_client(): void
+    {
+        $cookies = $this->container->get(CookieManager::class);
+
+        $this->regenerator->regenerate();
+
+        $this->assertSame(
+            (string) $this->session->id,
+            $cookies->get('tempest_session_id')?->value,
+        );
+    }
+}
diff --git a/tests/Integration/Http/SessionTest.php b/tests/Integration/Http/SessionTest.php
index e6d22a9107..7642f2a783 100644
--- a/tests/Integration/Http/SessionTest.php
+++ b/tests/Integration/Http/SessionTest.php
@@ -6,6 +6,7 @@
 
 use PHPUnit\Framework\Attributes\Test;
 use Tempest\Http\Session\Session;
+use Tempest\Http\Session\SessionId;
 use Tests\Tempest\Integration\FrameworkIntegrationTestCase;
 
 /**
@@ -139,6 +140,17 @@ public function clear(): void
         $this->assertEmpty($this->session->all());
     }
 
+    #[Test]
+    public function replace_id_preserves_data(): void
+    {
+        $this->session->set('key', 'value');
+
+        $this->session->replaceId(new SessionId('new_session'));
+
+        $this->assertSame('new_session', (string) $this->session->id);
+        $this->assertSame('value', $this->session->get('key'));
+    }
+
     #[Test]
     public function session_is_reset(): void
     {