diff --git a/src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java b/src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java
index 13b6606..85d2d7a 100644
--- a/src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java
+++ b/src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java
@@ -98,6 +98,18 @@ public class ExpenseListController {
         return new ResponseEntity<>(renamed, HttpStatus.OK);
     }
 
+    @PutMapping("/{id}/default-split")
+    public ResponseEntity<ExpenseList> updateDefaultSplit(@PathVariable("id") Long id,
+            @RequestBody @Valid DefaultSplitRequest request, Authentication authentication) {
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
+        Optional<ExpenseList> listOpt = expenseListService.findById(id);
+        if (listOpt.isEmpty())
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
+        assertMember(user, listOpt.get());
+        ExpenseList updated = expenseListService.updateDefaultSplit(id, request.getPercentage());
+        return new ResponseEntity<>(updated, HttpStatus.OK);
+    }
+
     @PostMapping("/{id}/add")
     public ResponseEntity<Expense> addExpenseToList(
             @PathVariable("id") Long expenseListId,
diff --git a/src/main/java/de/zendric/app/xpensely_server/model/DefaultSplitRequest.java b/src/main/java/de/zendric/app/xpensely_server/model/DefaultSplitRequest.java
new file mode 100644
index 0000000..6f10a88
--- /dev/null
+++ b/src/main/java/de/zendric/app/xpensely_server/model/DefaultSplitRequest.java
@@ -0,0 +1,17 @@
+package de.zendric.app.xpensely_server.model;
+
+import jakarta.validation.constraints.Max;
+import jakarta.validation.constraints.Min;
+import jakarta.validation.constraints.NotNull;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+@Data
+@NoArgsConstructor
+public class DefaultSplitRequest {
+
+    @NotNull(message = "Percentage is required")
+    @Min(value = 0, message = "Percentage must be between 0 and 100")
+    @Max(value = 100, message = "Percentage must be between 0 and 100")
+    private Integer percentage;
+}
diff --git a/src/main/java/de/zendric/app/xpensely_server/services/ExpenseListService.java b/src/main/java/de/zendric/app/xpensely_server/services/ExpenseListService.java
index e76c1b2..240528f 100644
--- a/src/main/java/de/zendric/app/xpensely_server/services/ExpenseListService.java
+++ b/src/main/java/de/zendric/app/xpensely_server/services/ExpenseListService.java
@@ -50,6 +50,13 @@ public class ExpenseListService {
         return repository.save(list);
     }
 
+    public ExpenseList updateDefaultSplit(Long id, int percentage) {
+        ExpenseList list = repository.findById(id)
+                .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + id));
+        list.setDefaultSplitPercentage(percentage);
+        return repository.save(list);
+    }
+
     public Optional<ExpenseList> findById(Long id) {
         return repository.findById(id);
     }
diff --git a/src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java b/src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java
index a4e0210..8da4b49 100644
--- a/src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java
+++ b/src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java
@@ -314,6 +314,25 @@ class ExpenseListControllerTest {
                 .andExpect(status().isNotFound());
     }
 
+    // --- Default split ---
+
+    @Test
+    void updateDefaultSplit_setsPercentageForMember() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L);
+        ExpenseList list = new ExpenseList(); list.setId(5L); list.setOwner(owner);
+
+        when(expenseListService.findById(5L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(owner);
+        when(expenseListService.updateDefaultSplit(5L, 60)).thenReturn(list);
+
+        mockMvc.perform(put("/api/expenselist/5/default-split")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"percentage\":60}"))
+                .andExpect(status().isOk());
+
+        org.mockito.Mockito.verify(expenseListService).updateDefaultSplit(5L, 60);
+    }
+
     // --- Duplicate-username invite guard ---
 
     @Test