diff --git a/app/routes/leagues/$leagueId.settings.tsx b/app/routes/leagues/$leagueId.settings.tsx
index 4d06100..37e1eaf 100644
--- a/app/routes/leagues/$leagueId.settings.tsx
+++ b/app/routes/leagues/$leagueId.settings.tsx
@@ -258,6 +258,14 @@ export async function action(args: Route.ActionArgs) {
       return { error: "Only admins can assign team owners" };
     }
 
+    // Check if user is already assigned to a team in this season
+    const teams = await findTeamsBySeasonId(season.id);
+    const userAlreadyHasTeam = teams.some(team => team.ownerId === userClerkId);
+    
+    if (userAlreadyHasTeam) {
+      return { error: "This user is already assigned to a team in this league" };
+    }
+
     try {
       await assignTeamOwner(teamId, userClerkId);
       return { success: true, message: "Owner assigned successfully" };
@@ -934,11 +942,20 @@ export default function LeagueSettings({ loaderData, actionData }: Route.Compone
                               <SelectValue placeholder="Select user" />
                             </SelectTrigger>
                             <SelectContent>
-                              {allUsers.map((user: any) => (
-                                <SelectItem key={user.id} value={user.clerkId}>
-                                  {user.displayName || user.username || user.email}
-                                </SelectItem>
-                              ))}
+                              {allUsers.map((user: any) => {
+                                // Check if user already owns a team in this league
+                                const userOwnsTeam = teams.some((t: any) => t.ownerId === user.clerkId);
+                                return (
+                                  <SelectItem 
+                                    key={user.id} 
+                                    value={user.clerkId}
+                                    disabled={userOwnsTeam}
+                                  >
+                                    {user.username || user.email}
+                                    {userOwnsTeam && " (already in league)"}
+                                  </SelectItem>
+                                );
+                              })}
                             </SelectContent>
                           </Select>
                           <Button
diff --git a/app/routes/leagues/__tests__/README.md b/app/routes/leagues/__tests__/README.md
index 2677d3f..c0f3158 100644
--- a/app/routes/leagues/__tests__/README.md
+++ b/app/routes/leagues/__tests__/README.md
@@ -74,7 +74,8 @@ The test suite covers:
 - ✅ Reject assignment for non-admin users
 - ✅ Allow commissioners to remove owners regardless of admin status
 
-### 5. Edge Cases (5 tests)
+### 5. Edge Cases (6 tests)
+- ✅ Prevent assigning user who already owns a team in the league
 - ✅ Handle removing owner from team with no owner
 - ✅ Handle assigning owner to team that already has owner
 - ✅ Handle invalid team ID gracefully
@@ -86,7 +87,7 @@ The test suite covers:
 - ✅ Reassigning owner from one user to another
 - ✅ Handle multiple teams with different ownership states
 
-**Total: 23 tests**
+**Total: 24 tests**
 
 ## Key Features Tested
 
diff --git a/app/routes/leagues/__tests__/team-management.test.ts b/app/routes/leagues/__tests__/team-management.test.ts
index edeadbf..c64bf58 100644
--- a/app/routes/leagues/__tests__/team-management.test.ts
+++ b/app/routes/leagues/__tests__/team-management.test.ts
@@ -317,6 +317,26 @@ describe('Team Management - Edge Cases', () => {
     vi.clearAllMocks();
   });
 
+  it('should prevent assigning user who already owns a team in the league', async () => {
+    const userClerkId = 'user-clerk-1';
+    
+    // Simulate that user already owns team-1
+    const existingTeams = [
+      createMockTeam({ id: 'team-1', ownerId: userClerkId, name: 'Team 1' }),
+      createMockTeam({ id: 'team-2', ownerId: null, name: 'Team 2' }),
+    ];
+
+    // Check if user already has a team
+    const userAlreadyHasTeam = existingTeams.some(team => team.ownerId === userClerkId);
+    
+    expect(userAlreadyHasTeam).toBe(true);
+    
+    // Should not call assignTeamOwner if user already has a team
+    if (userAlreadyHasTeam) {
+      expect(assignTeamOwner).not.toHaveBeenCalled();
+    }
+  });
+
   it('should handle removing owner from team that has no owner', async () => {
     const teamId = 'team-1';
     const mockTeam = createMockTeam({ id: teamId, ownerId: null, name: 'Team 1' });