diff --git a/src/main/java/com/thealgorithms/datastructures/trees/DiameterOfBinaryTree.java b/src/main/java/com/thealgorithms/datastructures/trees/DiameterOfBinaryTree.java
new file mode 100644
index 000000000000..36f09c79f94a
--- /dev/null
+++ b/src/main/java/com/thealgorithms/datastructures/trees/DiameterOfBinaryTree.java
@@ -0,0 +1,40 @@
+package com.thealgorithms.datastructures.trees;
+
+/**
+ * Computes the diameter of a binary tree — the length of the longest path
+ * between any two nodes (the path may or may not pass through the root).
+ *
+ * <p>Example:
+ * <pre>
+ *       1
+ *      / \
+ *     2   3
+ *    / \
+ *   4   5
+ * Diameter: 3 (path: 4 -> 2 -> 1 -> 3 or 5 -> 2 -> 1 -> 3)
+ * </pre>
+ *
+ * <p>Time complexity: O(n), Space complexity: O(h) where h is tree height.
+ *
+ * @see <a href="https://leetcode.com/problems/diameter-of-binary-tree/">LeetCode #543</a>
+ */
+public final class DiameterOfBinaryTree {
+    private DiameterOfBinaryTree() {
+    }
+
+    public static int diameterOfBinaryTree(BinaryTree.Node root) {
+        int[] maxDiameter = {0};
+        depth(root, maxDiameter);
+        return maxDiameter[0];
+    }
+
+    private static int depth(BinaryTree.Node node, int[] maxDiameter) {
+        if (node == null) {
+            return 0;
+        }
+        int leftDepth = depth(node.left, maxDiameter);
+        int rightDepth = depth(node.right, maxDiameter);
+        maxDiameter[0] = Math.max(maxDiameter[0], leftDepth + rightDepth);
+        return 1 + Math.max(leftDepth, rightDepth);
+    }
+}
diff --git a/src/test/java/com/thealgorithms/datastructures/trees/DiameterOfBinaryTreeTest.java b/src/test/java/com/thealgorithms/datastructures/trees/DiameterOfBinaryTreeTest.java
new file mode 100644
index 000000000000..d5cdfa1eee16
--- /dev/null
+++ b/src/test/java/com/thealgorithms/datastructures/trees/DiameterOfBinaryTreeTest.java
@@ -0,0 +1,47 @@
+package com.thealgorithms.datastructures.trees;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class DiameterOfBinaryTreeTest {
+
+    @Test
+    public void testNullRoot() {
+        assertEquals(0, DiameterOfBinaryTree.diameterOfBinaryTree(null));
+    }
+
+    @Test
+    public void testSingleNode() {
+        BinaryTree.Node root = TreeTestUtils.createTree(new Integer[] {1});
+        assertEquals(0, DiameterOfBinaryTree.diameterOfBinaryTree(root));
+    }
+
+    /*
+          1
+         / \
+        2   3
+       / \
+      4   5
+    Diameter: 3
+    */
+    @Test
+    public void testDiameterThroughRoot() {
+        BinaryTree.Node root = TreeTestUtils.createTree(new Integer[] {1, 2, 3, 4, 5});
+        assertEquals(3, DiameterOfBinaryTree.diameterOfBinaryTree(root));
+    }
+
+    /*
+        1
+       /
+      2
+     /
+    3
+    Diameter: 2
+    */
+    @Test
+    public void testLinearTree() {
+        BinaryTree.Node root = TreeTestUtils.createTree(new Integer[] {1, 2, null, 3});
+        assertEquals(2, DiameterOfBinaryTree.diameterOfBinaryTree(root));
+    }
+}