diff --git a/.changeset/quiet-keys-stay.md b/.changeset/quiet-keys-stay.md
new file mode 100644
index 000000000..4c7d4f804
--- /dev/null
+++ b/.changeset/quiet-keys-stay.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/form-core': patch
+---
+
+Fix: `formOptions.looseSchema` no longer turns optional schema properties into required ones, so the same schema can be passed to `validators[].run`.
diff --git a/packages/form-core/src/utils.public.ts b/packages/form-core/src/utils.public.ts
index 0955abaeb..4f5ed0cbf 100644
--- a/packages/form-core/src/utils.public.ts
+++ b/packages/form-core/src/utils.public.ts
@@ -35,8 +35,21 @@ export type InferUnion<TBase, TIncoming> = TBase extends BuiltInType
 type InferUnionObject<
   in out TBase extends object,
   in out TIncoming extends object,
-> = {
-  [K in keyof TBase | keyof TIncoming]: K extends keyof TBase
+> = InferUnionProps<
+  TBase,
+  TIncoming,
+  {
+    [K in keyof TBase as K extends keyof TIncoming ? never : K]: unknown
+  } & { [K in keyof TIncoming]: unknown }
+>
+
+// Mapping over `keyof TKeys` (a type parameter) is homomorphic, so each key
+// keeps the `?` modifier it has in `TKeys`. Keys that `TIncoming` declares take
+// its modifier, so a schema input that omits a property stays assignable.
+// Keys only in `TBase` keep theirs. Mapping over
+// `keyof TBase | keyof TIncoming` directly would drop every modifier.
+type InferUnionProps<TBase, TIncoming, TKeys> = {
+  [K in keyof TKeys]: K extends keyof TBase
     ? K extends keyof TIncoming
       ? InferUnion<TBase[K], TIncoming[K]>
       : TBase[K]
diff --git a/packages/form-core/tests/validation.test-d.ts b/packages/form-core/tests/validation.test-d.ts
index 7adec17de..4c8a259ff 100644
--- a/packages/form-core/tests/validation.test-d.ts
+++ b/packages/form-core/tests/validation.test-d.ts
@@ -263,6 +263,54 @@ describe('formOptions', () => {
     }>()
   })
 
+  it('keeps optional schema properties optional with a loose schema', () => {
+    const schema = z.object({
+      required: z.string(),
+      optional: z.string().optional(),
+      nested: z.object({
+        optional: z.number().optional(),
+      }),
+    })
+    const options = formOptions.looseSchema(schema, {
+      defaultValues: {},
+      validators: [
+        {
+          run: schema,
+          triggers: ['change'],
+        },
+      ],
+    })
+
+    expectTypeOf(options.defaultValues).toEqualTypeOf<{
+      required: string | undefined
+      optional?: string | undefined
+      nested:
+        | {
+            optional?: number | undefined
+          }
+        | undefined
+    }>()
+  })
+
+  it('keeps optional schema properties optional when defaults provide them', () => {
+    const schema = z.object({
+      optional: z.string().optional(),
+    })
+    const options = formOptions.looseSchema(schema, {
+      defaultValues: { optional: 'value' },
+      validators: [
+        {
+          run: schema,
+          triggers: ['change'],
+        },
+      ],
+    })
+
+    expectTypeOf(options.defaultValues).toEqualTypeOf<{
+      optional?: string | undefined
+    }>()
+  })
+
   it('types callback values when mixed with a loose schema', () => {
     type LooseValue = {
       name: string