diff --git a/Changes.md b/Changes.md
index a4e3153..060f028 100644
--- a/Changes.md
+++ b/Changes.md
@@ -2,6 +2,14 @@
 
 ---
 
+## 2026-04-27 – Feature: Neues Rechnungsnummernformat (PPPYYYYMM-nn)
+
+### Geändert
+- `src/Models/Invoice.php`: `generateRechnungsNr()` erzeugt jetzt das Format `PPPYYYYMM-nn` (Prefix + Jahr + Monat + zweistellige Sequenz, die bei Bedarf wächst); Zähler wird monatlich zurückgesetzt
+- `migrations/038_invoice_nr_month.sql`: Neue Spalte `invoice_nr_month` in `company_settings`
+
+---
+
 ## 2026-04-26 – Fix: Eigenanteil-Buchung als Einnahme statt Ausgabe
 
 ### Geändert
diff --git a/migrations/038_invoice_nr_month.sql b/migrations/038_invoice_nr_month.sql
new file mode 100644
index 0000000..b1be4ab
--- /dev/null
+++ b/migrations/038_invoice_nr_month.sql
@@ -0,0 +1,2 @@
+ALTER TABLE company_settings
+    ADD COLUMN IF NOT EXISTS invoice_nr_month TINYINT UNSIGNED NOT NULL DEFAULT 0;
diff --git a/src/Models/Invoice.php b/src/Models/Invoice.php
index 2660e53..8fbbc20 100644
--- a/src/Models/Invoice.php
+++ b/src/Models/Invoice.php
@@ -262,24 +262,26 @@ class Invoice
 
     public static function generateRechnungsNr(): string
     {
-        $settings = Database::fetchOne(
-            'SELECT invoice_nr_prefix, invoice_nr_next, invoice_nr_year FROM company_settings WHERE id=1'
+        $settings   = Database::fetchOne(
+            'SELECT invoice_nr_prefix, invoice_nr_next, invoice_nr_year, invoice_nr_month FROM company_settings WHERE id=1'
         );
-        $prefix    = $settings['invoice_nr_prefix'] ?? 'AR';
-        $next      = (int)($settings['invoice_nr_next'] ?? 1);
-        $savedYear = (int)($settings['invoice_nr_year'] ?? 0);
-        $year      = (int)date('Y');
+        $prefix     = $settings['invoice_nr_prefix'] ?? 'AR';
+        $next       = (int)($settings['invoice_nr_next'] ?? 1);
+        $savedYear  = (int)($settings['invoice_nr_year'] ?? 0);
+        $savedMonth = (int)($settings['invoice_nr_month'] ?? 0);
+        $year       = (int)date('Y');
+        $month      = (int)date('n');
 
-        // Reset counter on new year
-        if ($savedYear !== $year) {
+        if ($savedYear !== $year || $savedMonth !== $month) {
             $next = 1;
         }
 
-        $nr = sprintf('%s-%s-%04d', $prefix, $year, $next);
+        $digits = max(2, strlen((string)$next));
+        $nr = sprintf('%s%04d%02d-%0' . $digits . 'd', $prefix, $year, $month, $next);
 
         Database::execute(
-            'UPDATE company_settings SET invoice_nr_next=?, invoice_nr_year=? WHERE id=1',
-            [$next + 1, $year]
+            'UPDATE company_settings SET invoice_nr_next=?, invoice_nr_year=?, invoice_nr_month=? WHERE id=1',
+            [$next + 1, $year, $month]
         );
 
         return $nr;