diff --git a/app/Enums/RsvpStatus.php b/app/Enums/RsvpStatus.php
new file mode 100644
index 0000000..b10e3f3
--- /dev/null
+++ b/app/Enums/RsvpStatus.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Enums;
+
+/**
+ * RSVP-Status eines Nutzers für einen Meetup-Termin. `None` bildet den Zustand
+ * ab, dass der Nutzer in keiner der beiden Teilnehmer-Listen steht (= abgesagt
+ * bzw. nie zugesagt) und dient zugleich als Eingabewert zum Austragen.
+ */
+enum RsvpStatus: string
+{
+    case Attending = 'attending';
+    case Maybe = 'maybe';
+    case None = 'none';
+}
diff --git a/app/Http/Controllers/Api/MeetupEventController.php b/app/Http/Controllers/Api/MeetupEventController.php
index f6f34e5..0e1bf5f 100644
--- a/app/Http/Controllers/Api/MeetupEventController.php
+++ b/app/Http/Controllers/Api/MeetupEventController.php
@@ -3,11 +3,14 @@
 namespace App\Http\Controllers\Api;
 
 use App\Actions\MeetupEvents\CreateMeetupEventSeries;
+use App\Enums\RsvpStatus;
 use App\Http\Controllers\Controller;
+use App\Http\Requests\Api\RsvpMeetupEventRequest;
 use App\Http\Requests\Api\StoreMeetupEventRequest;
 use App\Http\Requests\Api\UpdateMeetupEventRequest;
 use App\Http\Resources\MeetupEventResource;
 use App\Models\MeetupEvent;
+use App\Models\User;
 use Carbon\Carbon;
 use Carbon\Exceptions\InvalidFormatException;
 use Dedoc\Scramble\Attributes\Group;
@@ -158,4 +161,49 @@ class MeetupEventController extends Controller
 
         return MeetupEventResource::make($meetupEvent);
     }
+
+    /**
+     * RSVP-Status eines Termins anzeigen
+     *
+     * Liefert den eigenen Teilnahme-Status des authentifizierten Nutzers für
+     * diesen Termin sowie die aktuellen Zähler der Zu- und Vielleicht-Sagen.
+     */
+    public function rsvpStatus(Request $request, MeetupEvent $meetupEvent): JsonResponse
+    {
+        return response()->json($this->rsvpPayload($meetupEvent, $request->user()));
+    }
+
+    /**
+     * Für einen Termin zu- oder absagen
+     *
+     * Trägt den authentifizierten Nutzer als Teilnehmer („attending"),
+     * Vielleicht-Teilnehmer („maybe") oder gar nicht („none", = absagen) ein.
+     * Der Anzeigename wird automatisch aus dem Profil übernommen. Idempotent:
+     * derselbe Status mehrfach gesetzt verändert nichts.
+     */
+    #[ResponseAttribute(status: 401, description: 'Nicht authentifiziert.')]
+    #[ResponseAttribute(status: 422, description: 'Validierungsfehler (unbekannter Status).')]
+    public function rsvp(RsvpMeetupEventRequest $request, MeetupEvent $meetupEvent): JsonResponse
+    {
+        $user = $request->user();
+        $status = RsvpStatus::from($request->validated('status'));
+
+        $meetupEvent->setRsvpFor($user, $status, (string) $user->name);
+
+        return response()->json($this->rsvpPayload($meetupEvent->fresh(), $user));
+    }
+
+    /**
+     * Einheitliche RSVP-Antwort: eigener Status + aktuelle Zähler.
+     *
+     * @return array{status: string, attendees: int, might_attendees: int}
+     */
+    private function rsvpPayload(MeetupEvent $meetupEvent, User $user): array
+    {
+        return [
+            'status' => $meetupEvent->rsvpStatusFor($user)->value,
+            'attendees' => count($meetupEvent->attendees ?? []),
+            'might_attendees' => count($meetupEvent->might_attendees ?? []),
+        ];
+    }
 }
diff --git a/app/Http/Requests/Api/RsvpMeetupEventRequest.php b/app/Http/Requests/Api/RsvpMeetupEventRequest.php
new file mode 100644
index 0000000..5becf2e
--- /dev/null
+++ b/app/Http/Requests/Api/RsvpMeetupEventRequest.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Http\Requests\Api;
+
+use App\Enums\RsvpStatus;
+use Illuminate\Foundation\Http\FormRequest;
+use Illuminate\Validation\Rule;
+
+class RsvpMeetupEventRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        // Jeder authentifizierte Nutzer darf für einen Termin zu-/absagen.
+        return $this->user() !== null;
+    }
+
+    /**
+     * @return array<string, array<int, mixed>>
+     */
+    public function rules(): array
+    {
+        return [
+            'status' => ['required', Rule::enum(RsvpStatus::class)],
+        ];
+    }
+}
diff --git a/app/Models/MeetupEvent.php b/app/Models/MeetupEvent.php
index ca1ee61..1cff8c6 100644
--- a/app/Models/MeetupEvent.php
+++ b/app/Models/MeetupEvent.php
@@ -3,11 +3,13 @@
 namespace App\Models;
 
 use App\Enums\RecurrenceType;
+use App\Enums\RsvpStatus;
 use App\Observers\MeetupEventObserver;
 use Illuminate\Database\Eloquent\Attributes\ObservedBy;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Support\Collection;
 
 #[ObservedBy([MeetupEventObserver::class])]
 class MeetupEvent extends Model
@@ -62,4 +64,76 @@ class MeetupEvent extends Model
     {
         return $this->belongsTo(Meetup::class);
     }
+
+    /**
+     * Eindeutige Kennung eines angemeldeten Nutzers in den Teilnehmer-Listen.
+     * Einträge werden als `id_<userId>|<name>` abgelegt; der angehängte Pipe
+     * grenzt z. B. `id_5` sauber von `id_50` ab.
+     */
+    public static function rsvpIdentifierFor(User $user): string
+    {
+        return 'id_'.$user->id;
+    }
+
+    /**
+     * Prefix, mit dem ein Eintrag des Nutzers in den Listen beginnt — inklusive
+     * Pipe, damit `id_5` nicht auf `id_50` matcht.
+     */
+    private static function rsvpPrefixFor(User $user): string
+    {
+        return self::rsvpIdentifierFor($user).'|';
+    }
+
+    /**
+     * Aktueller RSVP-Status des Nutzers für diesen Termin.
+     */
+    public function rsvpStatusFor(User $user): RsvpStatus
+    {
+        $prefix = self::rsvpPrefixFor($user);
+
+        if (collect($this->attendees ?? [])->contains(fn ($entry): bool => str($entry)->startsWith($prefix))) {
+            return RsvpStatus::Attending;
+        }
+
+        if (collect($this->might_attendees ?? [])->contains(fn ($entry): bool => str($entry)->startsWith($prefix))) {
+            return RsvpStatus::Maybe;
+        }
+
+        return RsvpStatus::None;
+    }
+
+    /**
+     * Setzt den RSVP-Status des Nutzers: entfernt ihn zunächst aus beiden Listen
+     * und trägt ihn anschließend in die gewählte Liste ein. `None` sagt nur ab
+     * (kein erneutes Eintragen). Persistiert die Änderung.
+     */
+    public function setRsvpFor(User $user, RsvpStatus $status, string $name): void
+    {
+        $prefix = self::rsvpPrefixFor($user);
+
+        $attendees = $this->withoutEntry($this->attendees, $prefix);
+        $mightAttendees = $this->withoutEntry($this->might_attendees, $prefix);
+
+        $entry = $prefix.$name;
+
+        match ($status) {
+            RsvpStatus::Attending => $attendees->push($entry),
+            RsvpStatus::Maybe => $mightAttendees->push($entry),
+            RsvpStatus::None => null,
+        };
+
+        $this->update([
+            'attendees' => $attendees->values()->all(),
+            'might_attendees' => $mightAttendees->values()->all(),
+        ]);
+    }
+
+    /**
+     * @param  array<int, string>|null  $list
+     * @return Collection<int, string>
+     */
+    private function withoutEntry(?array $list, string $prefix): Collection
+    {
+        return collect($list ?? [])->reject(fn ($entry): bool => str($entry)->startsWith($prefix));
+    }
 }
diff --git a/routes/api.php b/routes/api.php
index ee8a53a..590b9d6 100644
--- a/routes/api.php
+++ b/routes/api.php
@@ -85,6 +85,8 @@ Route::middleware('auth:sanctum')
         Route::patch('meetup-events/{meetupEvent}', [MeetupEventController::class, 'update'])->name('meetup-events.update');
         Route::get('my-meetup-events', [MeetupEventController::class, 'mine'])->name('meetup-events.mine');
         Route::get('my-meetup-events/{meetupEvent}', [MeetupEventController::class, 'mineShow'])->name('meetup-events.mine.show');
+        Route::get('meetup-events/{meetupEvent}/rsvp', [MeetupEventController::class, 'rsvpStatus'])->name('meetup-events.rsvp.show');
+        Route::post('meetup-events/{meetupEvent}/rsvp', [MeetupEventController::class, 'rsvp'])->name('meetup-events.rsvp');
     });
 
 Route::get('/lnurl-auth-callback', [LnurlAuthController::class, 'callback'])
diff --git a/tests/Feature/Api/MeetupEventRsvpApiTest.php b/tests/Feature/Api/MeetupEventRsvpApiTest.php
new file mode 100644
index 0000000..ca5dfbc
--- /dev/null
+++ b/tests/Feature/Api/MeetupEventRsvpApiTest.php
@@ -0,0 +1,84 @@
+<?php
+
+use App\Models\MeetupEvent;
+use App\Models\User;
+use Laravel\Sanctum\Sanctum;
+
+it('rejects a guest', function () {
+    $event = MeetupEvent::factory()->create();
+
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'attending'])
+        ->assertUnauthorized();
+});
+
+it('lets an authenticated user attend', function () {
+    Sanctum::actingAs($user = User::factory()->create(['name' => 'Satoshi']));
+    $event = MeetupEvent::factory()->create();
+
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'attending'])
+        ->assertSuccessful()
+        ->assertJson(['status' => 'attending', 'attendees' => 1, 'might_attendees' => 0]);
+
+    expect($event->fresh()->attendees)->toBe(["id_{$user->id}|Satoshi"]);
+});
+
+it('moves the user between lists without duplicating', function () {
+    Sanctum::actingAs($user = User::factory()->create(['name' => 'Satoshi']));
+    $event = MeetupEvent::factory()->create();
+
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'attending']);
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'maybe'])
+        ->assertJson(['status' => 'maybe', 'attendees' => 0, 'might_attendees' => 1]);
+
+    $event->refresh();
+    expect($event->attendees)->toBe([])
+        ->and($event->might_attendees)->toBe(["id_{$user->id}|Satoshi"]);
+});
+
+it('withdraws the user with status none', function () {
+    Sanctum::actingAs($user = User::factory()->create(['name' => 'Satoshi']));
+    $event = MeetupEvent::factory()->create();
+
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'attending']);
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'none'])
+        ->assertJson(['status' => 'none', 'attendees' => 0, 'might_attendees' => 0]);
+
+    expect($event->fresh()->attendees)->toBe([]);
+});
+
+it('keeps other attendees untouched', function () {
+    $event = MeetupEvent::factory()->create([
+        'attendees' => ['id_999|Hal', 'id_50|Nick'],
+    ]);
+    Sanctum::actingAs($user = User::factory()->create(['name' => 'Satoshi']));
+
+    // id_5 darf id_50 nicht versehentlich treffen (Prefix-Abgrenzung per Pipe).
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'attending'])
+        ->assertJson(['attendees' => 3]);
+
+    expect($event->fresh()->attendees)
+        ->toContain('id_999|Hal')
+        ->toContain('id_50|Nick')
+        ->toContain("id_{$user->id}|Satoshi");
+});
+
+it('reports the current status and counts', function () {
+    Sanctum::actingAs($user = User::factory()->create(['name' => 'Satoshi']));
+    $event = MeetupEvent::factory()->create([
+        'attendees' => ['id_999|Hal'],
+        'might_attendees' => ["id_{$user->id}|Satoshi"],
+    ]);
+
+    $this->getJson("/api/meetup-events/{$event->id}/rsvp")
+        ->assertSuccessful()
+        ->assertJson(['status' => 'maybe', 'attendees' => 1, 'might_attendees' => 1]);
+});
+
+it('validates the status value', function () {
+    Sanctum::actingAs(User::factory()->create());
+    $event = MeetupEvent::factory()->create();
+
+    $this->postJson("/api/meetup-events/{$event->id}/rsvp", ['status' => 'bogus'])
+        ->assertUnprocessable()
+        ->assertJsonValidationErrors(['status']);
+});