diff --git a/system/Session/Session.php b/system/Session/Session.php
index 0540c1508a9e..9cf65f3cde25 100644
--- a/system/Session/Session.php
+++ b/system/Session/Session.php
@@ -817,39 +817,31 @@ public function removeTempdata(string $key)
      */
     public function markAsTempdata($key, int $ttl = 300): bool
     {
-        $ttl += Time::now()->getTimestamp();
+        $time = Time::now()->getTimestamp();
+        $keys = is_array($key) ? $key : [$key];
 
-        if (is_array($key)) {
-            $temp = [];
-
-            foreach ($key as $k => $v) {
-                // Do we have a key => ttl pair, or just a key?
-                if (is_int($k)) {
-                    $k = $v;
-                    $v = $ttl;
-                } elseif (is_string($v)) {
-                    $v = Time::now()->getTimestamp() + $ttl;
-                } else {
-                    $v += Time::now()->getTimestamp();
-                }
+        if (array_is_list($keys)) {
+            $keys = array_fill_keys($keys, $ttl);
+        }
 
-                if (! array_key_exists($k, $_SESSION)) {
-                    return false;
-                }
+        $tempdata = [];
 
-                $temp[$k] = $v;
+        foreach ($keys as $sessionKey => $timeToLive) {
+            if (! array_key_exists($sessionKey, $_SESSION)) {
+                return false;
             }
 
-            $_SESSION['__ci_vars'] = isset($_SESSION['__ci_vars']) ? array_merge($_SESSION['__ci_vars'], $temp) : $temp;
-
-            return true;
-        }
+            if (is_int($timeToLive)) {
+                $timeToLive += $time;
+            } else {
+                $timeToLive = $time + $ttl;
+            }
 
-        if (! isset($_SESSION[$key])) {
-            return false;
+            $tempdata[$sessionKey] = $timeToLive;
         }
 
-        $_SESSION['__ci_vars'][$key] = $ttl;
+        $_SESSION['__ci_vars'] ??= [];
+        $_SESSION['__ci_vars'] = [...$_SESSION['__ci_vars'], ...$tempdata];
 
         return true;
     }
diff --git a/tests/system/Session/SessionTest.php b/tests/system/Session/SessionTest.php
index 7a8de643cc35..d469c595cd4f 100644
--- a/tests/system/Session/SessionTest.php
+++ b/tests/system/Session/SessionTest.php
@@ -476,6 +476,23 @@ public function testSetTempDataArraySingleTTL(): void
         $this->assertLessThanOrEqual($_SESSION['__ci_vars']['baz'], $time + 200);
     }
 
+    /**
+     * @see https://github.com/codeigniter4/CodeIgniter4/issues/9534
+     */
+    public function testSetTempDataOnArrayData(): void
+    {
+        $session = $this->getInstance();
+        $session->start();
+
+        $time = time();
+
+        $session->setTempdata(['foo1' => 'bar1'], null, 200);
+        $session->setTempdata('foo2', 'bar2', 200);
+
+        $this->assertLessThanOrEqual($_SESSION['__ci_vars']['foo1'], $time + 200);
+        $this->assertLessThanOrEqual($_SESSION['__ci_vars']['foo2'], $time + 200);
+    }
+
     /**
      * @see https://github.com/codeigniter4/CodeIgniter4/pull/9536#discussion_r2051798869
      */
diff --git a/user_guide_src/source/changelogs/v4.6.1.rst b/user_guide_src/source/changelogs/v4.6.1.rst
index 2eb8db9600f3..8a327aea5f9e 100644
--- a/user_guide_src/source/changelogs/v4.6.1.rst
+++ b/user_guide_src/source/changelogs/v4.6.1.rst
@@ -40,6 +40,7 @@ Bugs Fixed
 - **Database:** Fixed a bug in ``Postgre`` and ``SQLite3`` handlers where composite unique keys were not fully taken into account for ``upsert`` type of queries.
 - **Database:** Fixed a bug in the ``OCI8`` and ``SQLSRV`` drivers where ``getVersion()`` returned an empty string when the database connection was not yet established.
 - **Logger:** Fixed a bug where the ``{line}`` variable couldn't be used without specifying the ``{file}`` variable when logging the message.
+- **Session:** Fixed a bug where ``Session::markAsTempdata()`` would give the wrong TTL when array data is supplied to ``Session::setTempdata()``.
 - **Toolbar:** Fixed a bug where setting ``maxHistory`` to ``0`` would produce a JavaScript error in the Debug Toolbar.
 - **Toolbar:** Fixed a bug where setting ``maxHistory`` to ``0`` prevented log files from being properly cleared.