diff --git a/migrations/033_customer_nr_prefix.sql b/migrations/033_customer_nr_prefix.sql
new file mode 100644
index 0000000..dc1012c
--- /dev/null
+++ b/migrations/033_customer_nr_prefix.sql
@@ -0,0 +1,5 @@
+-- Migration 033: Kunden-Nummernpräfix konfigurierbar machen
+ALTER TABLE company_settings
+    ADD COLUMN IF NOT EXISTS customer_nr_prefix VARCHAR(10) NOT NULL DEFAULT 'K-'
+        COMMENT 'Präfix für Kundennummern (z.B. K-)'
+        AFTER anlage_nr_prefix;
diff --git a/src/Models/Customer.php b/src/Models/Customer.php
index 255bab5..1d6f027 100644
--- a/src/Models/Customer.php
+++ b/src/Models/Customer.php
@@ -108,8 +108,16 @@ class Customer
 
     public static function generateKundennummer(): string
     {
-        $result = Database::fetchOne('SELECT MAX(CAST(SUBSTRING(kundennummer, 3) AS UNSIGNED)) as max_nr FROM customers WHERE kundennummer REGEXP \'^KD[0-9]+$\'');
+        $settings = \Steuerkram\Models\Setting::get();
+        $prefix   = $settings['customer_nr_prefix'] ?? 'K-';
+        $prefixLen = strlen($prefix) + 1;
+        $escaped  = preg_quote($prefix, '/');
+        $result   = Database::fetchOne(
+            "SELECT MAX(CAST(SUBSTRING(kundennummer, {$prefixLen}) AS UNSIGNED)) as max_nr
+             FROM customers WHERE kundennummer REGEXP ?",
+            ['^' . $escaped . '[0-9]+$']
+        );
         $next = (int)($result['max_nr'] ?? 0) + 1;
-        return sprintf('KD%04d', $next);
+        return $prefix . sprintf('%04d', $next);
     }
 }
diff --git a/src/Models/Setting.php b/src/Models/Setting.php
index e610d29..85339a4 100644
--- a/src/Models/Setting.php
+++ b/src/Models/Setting.php
@@ -32,7 +32,7 @@ class Setting
                 ust_id=?, steuernummer=?, finanzamt=?,
                 iban=?, bic=?, bank_name=?,
                 ustva_steuernummer=?, ustva_bundesland=?,
-                invoice_nr_prefix=?, beleg_nr_prefix=?, anlage_nr_prefix=?,
+                invoice_nr_prefix=?, beleg_nr_prefix=?, anlage_nr_prefix=?, customer_nr_prefix=?,
                 payment_days_default=?, test_mode=?, ki_vorschlag_aktiv=?,
                 nextcloud_url=?, nextcloud_user=?, nextcloud_pass=?,
                 nextcloud_folder=?, nextcloud_done=?,
@@ -59,6 +59,7 @@ class Setting
                 $data['invoice_nr_prefix'] ?? 'AR',
                 $data['beleg_nr_prefix'] ?? 'BLG',
                 $data['anlage_nr_prefix'] ?? 'ANL',
+                $data['customer_nr_prefix'] ?? 'K-',
                 (int)($data['payment_days_default'] ?? 14),
                 isset($data['test_mode']) ? 1 : 0,
                 isset($data['ki_vorschlag_aktiv']) ? 1 : 0,
diff --git a/src/Views/settings/form.php b/src/Views/settings/form.php
index 35d474d..96b5e3b 100644
--- a/src/Views/settings/form.php
+++ b/src/Views/settings/form.php
@@ -236,6 +236,10 @@
                         <label>Anlagen-Präfix</label>
                         <input type="text" name="anlage_nr_prefix" value="<?= $h($settings['anlage_nr_prefix'] ?? 'ANL') ?>" maxlength="10">
                     </div>
+                    <div class="form-group">
+                        <label>Kunden-Präfix</label>
+                        <input type="text" name="customer_nr_prefix" value="<?= $h($settings['customer_nr_prefix'] ?? 'K-') ?>" maxlength="10">
+                    </div>
                 </div>
                 <div class="form-group">
                     <label>Standard-Zahlungsziel (Tage)</label>
diff --git a/tests/23_KundenNummernkreisTest.php b/tests/23_KundenNummernkreisTest.php
new file mode 100644
index 0000000..48d920f
--- /dev/null
+++ b/tests/23_KundenNummernkreisTest.php
@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+use Steuerkram\Models\Customer;
+use Steuerkram\Models\Setting;
+use Steuerkram\Database;
+
+class KundenNummernkreisTest extends TestCase
+{
+    protected string $testName = 'Test 23: Kunden-Nummernkreis konfigurierbar';
+
+    public function run(): void
+    {
+        $settings = Setting::get();
+        $this->assert(array_key_exists('customer_nr_prefix', $settings), 'customer_nr_prefix-Spalte existiert in company_settings');
+
+        // Präfix in den Einstellungen speichern und lesen
+        $originalPrefix = $settings['customer_nr_prefix'] ?? 'K-';
+        $testPrefix = 'TEST-';
+        Setting::update(array_merge($settings, ['customer_nr_prefix' => $testPrefix]));
+        $updated = Setting::get();
+        $this->assertEquals($testPrefix, $updated['customer_nr_prefix'], 'customer_nr_prefix wird korrekt gespeichert');
+
+        // Kundennummer mit neuem Präfix generieren
+        $nr = Customer::generateKundennummer();
+        $this->assert(str_starts_with($nr, $testPrefix), 'Generierte Kundennummer beginnt mit neuem Präfix');
+        $numericPart = substr($nr, strlen($testPrefix));
+        $this->assert(ctype_digit($numericPart), 'Numerischer Teil der Kundennummer enthält nur Ziffern');
+        $this->assert((int)$numericPart > 0, 'Numerischer Teil > 0');
+
+        // Zurücksetzen auf Standard-Präfix
+        Setting::update(array_merge($updated, ['customer_nr_prefix' => 'K-']));
+        $reset = Setting::get();
+        $this->assertEquals('K-', $reset['customer_nr_prefix'], 'Präfix auf K- zurückgesetzt');
+
+        // Kundennummer mit Default-Präfix K-
+        $nrDefault = Customer::generateKundennummer();
+        $this->assert(str_starts_with($nrDefault, 'K-'), 'Generierte Kundennummer beginnt mit K-');
+
+        // Originalwert wiederherstellen
+        Setting::update(array_merge($reset, ['customer_nr_prefix' => $originalPrefix]));
+    }
+}