diff --git a/app/Exceptions/Api/TimeEntryMemberCanNotBeChangedWhileRunningApiException.php b/app/Exceptions/Api/TimeEntryMemberCanNotBeChangedWhileRunningApiException.php
new file mode 100644
index 00000000..b39367d7
--- /dev/null
+++ b/app/Exceptions/Api/TimeEntryMemberCanNotBeChangedWhileRunningApiException.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Exceptions\Api;
+
+class TimeEntryMemberCanNotBeChangedWhileRunningApiException extends ApiException
+{
+    public const string KEY = 'time_entry_member_can_not_be_changed_while_running';
+
+    public function report(): bool
+    {
+        return true;
+    }
+}
diff --git a/app/Http/Controllers/Api/V1/TimeEntryController.php b/app/Http/Controllers/Api/V1/TimeEntryController.php
index 8cdfa8cb..90f6e327 100644
--- a/app/Http/Controllers/Api/V1/TimeEntryController.php
+++ b/app/Http/Controllers/Api/V1/TimeEntryController.php
@@ -11,6 +11,7 @@ use App\Exceptions\Api\FeatureIsNotAvailableInFreePlanApiException;
 use App\Exceptions\Api\OverlappingTimeEntryApiException;
 use App\Exceptions\Api\PdfRendererIsNotConfiguredException;
 use App\Exceptions\Api\TimeEntryCanNotBeRestartedApiException;
+use App\Exceptions\Api\TimeEntryMemberCanNotBeChangedWhileRunningApiException;
 use App\Exceptions\Api\TimeEntryStillRunningApiException;
 use App\Http\Requests\V1\TimeEntry\TimeEntryAggregateExportRequest;
 use App\Http\Requests\V1\TimeEntry\TimeEntryAggregateRequest;
@@ -668,7 +669,7 @@ class TimeEntryController extends Controller
      *
      * To stop a running timer, set `end` to the stop time (UTC). Times the user gives in their own timezone must be converted to UTC first.
      *
-     * @throws AuthorizationException|TimeEntryCanNotBeRestartedApiException
+     * @throws AuthorizationException|TimeEntryCanNotBeRestartedApiException|TimeEntryMemberCanNotBeChangedWhileRunningApiException
      *
      * @operationId updateTimeEntry
      */
@@ -694,6 +695,11 @@ class TimeEntryController extends Controller
         $effectiveEnd = $request->has('end') ? ($request->input('end') !== null ? Carbon::parse($request->input('end')) : null) : $timeEntry->end;
         $this->assertNoOverlap($organization, $effectiveMember, $effectiveStart, $effectiveEnd, $timeEntry);
 
+        // The new member could already have a running time entry
+        if ($effectiveEnd === null && $newMember !== null && $newMember->getKey() !== $timeEntry->member_id) {
+            throw new TimeEntryMemberCanNotBeChangedWhileRunningApiException;
+        }
+
         $oldProject = $timeEntry->project;
         $oldTask = $timeEntry->task;
 
@@ -819,6 +825,12 @@ class TimeEntryController extends Controller
 
                 continue;
             }
+            // The new member could already have a running time entry
+            if ($newMember !== null && $timeEntry->end === null && $newMember->getKey() !== $timeEntry->member_id) {
+                $error->push($id);
+
+                continue;
+            }
             $oldProject = $timeEntry->project;
             $oldTask = $timeEntry->task;
 
diff --git a/lang/en/exceptions.php b/lang/en/exceptions.php
index 485eb145..110125b0 100644
--- a/lang/en/exceptions.php
+++ b/lang/en/exceptions.php
@@ -19,6 +19,7 @@ use App\Exceptions\Api\PdfRendererIsNotConfiguredException;
 use App\Exceptions\Api\PersonalAccessClientIsNotConfiguredException;
 use App\Exceptions\Api\ThisPlaceholderCanNotBeInvitedUseTheMergeToolInsteadException;
 use App\Exceptions\Api\TimeEntryCanNotBeRestartedApiException;
+use App\Exceptions\Api\TimeEntryMemberCanNotBeChangedWhileRunningApiException;
 use App\Exceptions\Api\TimeEntryStillRunningApiException;
 use App\Exceptions\Api\UserIsAlreadyMemberOfOrganizationApiException;
 use App\Exceptions\Api\UserIsAlreadyMemberOfProjectApiException;
@@ -31,6 +32,7 @@ return [
         TimeEntryStillRunningApiException::KEY => 'Time entry is still running',
         UserNotPlaceholderApiException::KEY => 'The given user is not a placeholder',
         TimeEntryCanNotBeRestartedApiException::KEY => 'Time entry is already stopped and can not be restarted',
+        TimeEntryMemberCanNotBeChangedWhileRunningApiException::KEY => 'The member of a running time entry can not be changed',
         InactiveUserCanNotBeUsedApiException::KEY => 'Inactive user can not be used',
         UserIsAlreadyMemberOfOrganizationApiException::KEY => 'User is already a member of the organization',
         UserIsAlreadyMemberOfProjectApiException::KEY => 'User is already a member of the project',
diff --git a/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php b/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
index 459d2d68..cfa79fff 100644
--- a/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
+++ b/tests/Unit/Endpoint/Api/V1/TimeEntryEndpointTest.php
@@ -3237,6 +3237,80 @@ class TimeEntryEndpointTest extends ApiEndpointTestAbstract
         ]);
     }
 
+    public function test_update_endpoint_fails_if_member_of_running_time_entry_is_changed(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:all',
+        ]);
+        $otherMember = Member::factory()->forOrganization($data->organization)->role(Role::Employee)->create();
+        $timeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->active()->create();
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $timeEntry->getKey()]), [
+            'member_id' => $otherMember->getKey(),
+        ]);
+
+        // Assert
+        $response->assertStatus(400);
+        $response->assertJsonPath('key', 'time_entry_member_can_not_be_changed_while_running');
+        $this->assertDatabaseHas(TimeEntry::class, [
+            'id' => $timeEntry->getKey(),
+            'member_id' => $data->member->getKey(),
+            'user_id' => $data->user->getKey(),
+        ]);
+    }
+
+    public function test_update_endpoint_can_update_running_time_entry_with_unchanged_member_id(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:own',
+        ]);
+        $timeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->active()->create();
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $timeEntry->getKey()]), [
+            'member_id' => $data->member->getKey(),
+            'description' => 'New description',
+        ]);
+
+        // Assert
+        $response->assertStatus(200);
+        $this->assertDatabaseHas(TimeEntry::class, [
+            'id' => $timeEntry->getKey(),
+            'description' => 'New description',
+        ]);
+    }
+
+    public function test_update_endpoint_can_stop_running_time_entry_and_change_member_in_the_same_request(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:all',
+        ]);
+        $otherUser = User::factory()->create();
+        $otherMember = Member::factory()->forOrganization($data->organization)->forUser($otherUser)->role(Role::Employee)->create();
+        $timeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->active()->create();
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->putJson(route('api.v1.time-entries.update', [$data->organization->getKey(), $timeEntry->getKey()]), [
+            'member_id' => $otherMember->getKey(),
+            'end' => Carbon::now()->toIso8601ZuluString(),
+        ]);
+
+        // Assert
+        $response->assertStatus(200);
+        $this->assertDatabaseHas(TimeEntry::class, [
+            'id' => $timeEntry->getKey(),
+            'member_id' => $otherMember->getKey(),
+            'user_id' => $otherUser->getKey(),
+        ]);
+    }
+
     public function test_update_endpoint_can_update_project_and_automatically_set_client(): void
     {
         // Arrange
@@ -4104,6 +4178,51 @@ class TimeEntryEndpointTest extends ApiEndpointTestAbstract
         ]);
     }
 
+    public function test_update_multiple_does_not_change_member_of_running_time_entries(): void
+    {
+        // Arrange
+        $data = $this->createUserWithPermission([
+            'time-entries:update:all',
+        ]);
+        $otherMember = Member::factory()->forOrganization($data->organization)->role(Role::Employee)->create();
+        $runningTimeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->active()->create();
+        $runningTimeEntryOfOtherMember = TimeEntry::factory()->forOrganization($data->organization)->forMember($otherMember)->active()->create();
+        $finishedTimeEntry = TimeEntry::factory()->forOrganization($data->organization)->forMember($data->member)->create();
+        Passport::actingAs($data->user);
+
+        // Act
+        $response = $this->patchJson(route('api.v1.time-entries.update-multiple', [$data->organization->getKey()]), [
+            'ids' => [
+                $runningTimeEntry->getKey(),
+                $runningTimeEntryOfOtherMember->getKey(),
+                $finishedTimeEntry->getKey(),
+            ],
+            'changes' => [
+                'member_id' => $otherMember->getKey(),
+            ],
+        ]);
+
+        // Assert
+        $response->assertStatus(200);
+        $response->assertExactJson([
+            'success' => [
+                $runningTimeEntryOfOtherMember->getKey(),
+                $finishedTimeEntry->getKey(),
+            ],
+            'error' => [
+                $runningTimeEntry->getKey(),
+            ],
+        ]);
+        $this->assertDatabaseHas(TimeEntry::class, [
+            'id' => $runningTimeEntry->getKey(),
+            'member_id' => $data->member->getKey(),
+        ]);
+        $this->assertDatabaseHas(TimeEntry::class, [
+            'id' => $finishedTimeEntry->getKey(),
+            'member_id' => $otherMember->getKey(),
+        ]);
+    }
+
     public function test_update_multiple_updates_user_id_when_member_id_changes(): void
     {
         // Arrange
diff --git a/tests/Unit/Exceptions/Api/ApiExceptionTest.php b/tests/Unit/Exceptions/Api/ApiExceptionTest.php
index e52bff0d..c698913f 100644
--- a/tests/Unit/Exceptions/Api/ApiExceptionTest.php
+++ b/tests/Unit/Exceptions/Api/ApiExceptionTest.php
@@ -22,6 +22,7 @@ use App\Exceptions\Api\PdfRendererIsNotConfiguredException;
 use App\Exceptions\Api\PersonalAccessClientIsNotConfiguredException;
 use App\Exceptions\Api\ThisPlaceholderCanNotBeInvitedUseTheMergeToolInsteadException;
 use App\Exceptions\Api\TimeEntryCanNotBeRestartedApiException;
+use App\Exceptions\Api\TimeEntryMemberCanNotBeChangedWhileRunningApiException;
 use App\Exceptions\Api\TimeEntryStillRunningApiException;
 use App\Exceptions\Api\UserIsAlreadyMemberOfOrganizationApiException;
 use App\Exceptions\Api\UserIsAlreadyMemberOfProjectApiException;
@@ -53,6 +54,7 @@ class ApiExceptionTest extends TestCase
         yield OverlappingTimeEntryApiException::class => [new OverlappingTimeEntryApiException];
         yield ThisPlaceholderCanNotBeInvitedUseTheMergeToolInsteadException::class => [new ThisPlaceholderCanNotBeInvitedUseTheMergeToolInsteadException];
         yield TimeEntryCanNotBeRestartedApiException::class => [new TimeEntryCanNotBeRestartedApiException];
+        yield TimeEntryMemberCanNotBeChangedWhileRunningApiException::class => [new TimeEntryMemberCanNotBeChangedWhileRunningApiException];
         yield TimeEntryStillRunningApiException::class => [new TimeEntryStillRunningApiException];
         yield UserIsAlreadyMemberOfOrganizationApiException::class => [new UserIsAlreadyMemberOfOrganizationApiException];
         yield UserIsAlreadyMemberOfProjectApiException::class => [new UserIsAlreadyMemberOfProjectApiException];