diff --git a/system/Session/Handlers/MemcachedHandler.php b/system/Session/Handlers/MemcachedHandler.php
index c64ba65f6454..7b43bc31bda4 100644
--- a/system/Session/Handlers/MemcachedHandler.php
+++ b/system/Session/Handlers/MemcachedHandler.php
@@ -221,11 +221,7 @@ public function write($id, $data): bool
     public function close(): bool
     {
         if (isset($this->memcached)) {
-            if (isset($this->lockKey)) {
-                $this->memcached->delete($this->lockKey);
-            }
-
-            return true;
+            return ! isset($this->lockKey) || $this->releaseLock();
         }
 
         return false;
@@ -265,11 +261,15 @@ public function gc($max_lifetime): int
      */
     protected function lockSession(string $sessionID): bool
     {
-        if (isset($this->lockKey)) {
+        $lockKey = $this->keyPrefix . $sessionID . ':lock';
+
+        // PHP reuses the SessionHandler object on regeneration,
+        // so we need to check here if the lock key is for the
+        // correct session ID.
+        if ($this->lockKey === $lockKey) {
             return $this->memcached->replace($this->lockKey, Time::now()->getTimestamp(), 300);
         }
 
-        $lockKey = $this->keyPrefix . $sessionID . ':lock';
         $attempt = 0;
 
         do {
diff --git a/tests/system/Session/Handlers/MemcachedHandlerTest.php b/tests/system/Session/Handlers/MemcachedHandlerTest.php
index 8a24a7377722..d00489d61ce5 100644
--- a/tests/system/Session/Handlers/MemcachedHandlerTest.php
+++ b/tests/system/Session/Handlers/MemcachedHandlerTest.php
@@ -18,6 +18,7 @@
 use CodeIgniter\Test\TestLogger;
 use Config\Logger as LoggerConfig;
 use Config\Session as SessionConfig;
+use Memcached;
 use PHPUnit\Framework\Attributes\Group;
 use PHPUnit\Framework\Attributes\RequiresPhpExtension;
 
@@ -33,6 +34,11 @@ final class MemcachedHandlerTest extends CIUnitTestCase
     private string $sessionSavePath = '127.0.0.1:11211';
     private string $userIpAddress   = '127.0.0.1';
 
+    /**
+     * @var list<string>
+     */
+    private array $sessionIds = [];
+
     /**
      * @param array<string, bool|int|string|null> $options Replace values for `Config\Session`.
      */
@@ -62,11 +68,34 @@ protected function getInstance($options = []): MemcachedHandler
 
     protected function tearDown(): void
     {
+        if ($this->sessionIds !== []) {
+            $memcached = new Memcached();
+            $memcached->setOption(Memcached::OPT_BINARY_PROTOCOL, true);
+            $memcached->addServer('127.0.0.1', 11211);
+
+            $prefix = 'ci_session:' . $this->sessionName . ':';
+
+            foreach ($this->sessionIds as $sessionId) {
+                $memcached->delete($prefix . $sessionId);
+                $memcached->delete($prefix . $sessionId . ':lock');
+            }
+
+            $this->sessionIds = [];
+        }
+
         parent::tearDown();
 
         MemcachedHandler::resetPersistentConnections();
     }
 
+    private function generateSessionId(string $prefix): string
+    {
+        $sessionId          = $prefix . bin2hex(random_bytes(13));
+        $this->sessionIds[] = $sessionId;
+
+        return $sessionId;
+    }
+
     public function testConstructorThrowsWithEmptySavePath(): void
     {
         $this->expectException(SessionException::class);
@@ -128,4 +157,78 @@ public function testGC(): void
         $handler = $this->getInstance();
         $this->assertSame(1, $handler->gc(3600));
     }
+
+    public function testReopenAndReadWithSameHandlerInstance(): void
+    {
+        $handler = $this->getInstance();
+        $handler->open($this->sessionSavePath, $this->sessionName);
+
+        $sessionId = $this->generateSessionId('666666');
+
+        $this->assertSame('', $handler->read($sessionId));
+
+        $data = '__ci_last_regenerate|i:1664607454;key|s:5:"value";';
+        $this->assertTrue($handler->write($sessionId, $data));
+        $this->assertTrue($handler->close());
+
+        // Reopen with the same handler instance (e.g. session_write_close() + session_start())
+        $this->assertTrue($handler->open($this->sessionSavePath, $this->sessionName));
+        $this->assertSame($data, $handler->read($sessionId));
+        $this->assertTrue($handler->close());
+    }
+
+    public function testRegenerateSessionIdWithSameHandlerInstance(): void
+    {
+        $handler = $this->getInstance();
+        $handler->open($this->sessionSavePath, $this->sessionName);
+
+        $oldSessionId = $this->generateSessionId('777777');
+        $newSessionId = $this->generateSessionId('888888');
+
+        $this->assertSame('', $handler->read($oldSessionId));
+
+        $data = '__ci_last_regenerate|i:1664607454;role|s:5:"admin";';
+        $this->assertTrue($handler->write($oldSessionId, $data));
+        $this->assertTrue($handler->close());
+
+        // PHP reuses the same SessionHandler object on regeneration
+        $this->assertTrue($handler->open($this->sessionSavePath, $this->sessionName));
+        $this->assertSame('', $handler->read($newSessionId));
+        $this->assertTrue($handler->write($newSessionId, $data));
+        $this->assertTrue($handler->close());
+
+        $this->assertTrue($handler->open($this->sessionSavePath, $this->sessionName));
+        $this->assertSame($data, $handler->read($newSessionId));
+        $this->assertTrue($handler->close());
+    }
+
+    public function testRegenerateSessionIdWriteWithoutClose(): void
+    {
+        $handler = $this->getInstance();
+        $handler->open($this->sessionSavePath, $this->sessionName);
+
+        $oldSessionId = $this->generateSessionId('999999');
+        $newSessionId = $this->generateSessionId('aaaaaa');
+
+        $this->assertSame('', $handler->read($oldSessionId));
+
+        $oldData = '__ci_last_regenerate|i:1664607454;role|s:5:"guest";';
+        $this->assertTrue($handler->write($oldSessionId, $oldData));
+
+        // During PHP session_regenerate_id(), write() is called with the new ID
+        // while the lock for the old session ID is still held.
+        $newData = '__ci_last_regenerate|i:1664607500;role|s:5:"admin";';
+        $this->assertTrue($handler->write($newSessionId, $newData));
+        $this->assertTrue($handler->close());
+
+        // Verify the old lock was released and the new session data is readable
+        $handler2 = $this->getInstance();
+        $this->assertTrue($handler2->open($this->sessionSavePath, $this->sessionName));
+        $this->assertSame($oldData, $handler2->read($oldSessionId));
+        $this->assertTrue($handler2->close());
+
+        $this->assertTrue($handler2->open($this->sessionSavePath, $this->sessionName));
+        $this->assertSame($newData, $handler2->read($newSessionId));
+        $this->assertTrue($handler2->close());
+    }
 }