diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameGenerator.java b/service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameGenerator.java
index a941bff25..222d00954 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameGenerator.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameGenerator.java
@@ -70,7 +70,7 @@ public class UsernameGenerator {
       // check discriminators of the current width up to attemptsPerWidth times
       for (int i = 0; i < attemptsPerWidth; i++) {
         int discriminator = ThreadLocalRandom.current().nextInt(rangeMin, rangeMax);
-        String username = UsernameGenerator.fromParts(nickname, discriminator);
+        String username = fromParts(nickname, discriminator);
         attempts++;
         if (usernameAvailableFun.test(username)) {
           DISCRIMINATOR_ATTEMPT_COUNTER.record(attempts);
@@ -101,11 +101,12 @@ public class UsernameGenerator {
   /**
    * Generate a username from a nickname and discriminator
    */
-  public static String fromParts(final String nickname, final int discriminator) throws IllegalArgumentException {
+  public String fromParts(final String nickname, final int discriminator) throws IllegalArgumentException {
     if (!isValidNickname(nickname)) {
       throw new IllegalArgumentException("Invalid nickname " + nickname);
     }
-    return nickname + SEPARATOR + discriminator;
+    // zero pad discriminators less than the discriminator initial width
+    return String.format("%s#%0" + initialWidth + "d", nickname, discriminator);
   }
 
   public static boolean isValidNickname(final String nickname) {
diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerUsernameIntegrationTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerUsernameIntegrationTest.java
index 8bbf72725..136bc16b5 100644
--- a/service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerUsernameIntegrationTest.java
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/storage/AccountsManagerUsernameIntegrationTest.java
@@ -64,6 +64,7 @@ class AccountsManagerUsernameIntegrationTest {
 
   private AccountsManager accountsManager;
   private Accounts accounts;
+  private UsernameGenerator usernameGenerator;
 
   @BeforeEach
   void setup() throws InterruptedException {
@@ -141,6 +142,7 @@ class AccountsManagerUsernameIntegrationTest {
     when(experimentEnrollmentManager.isEnrolled(any(UUID.class), eq(AccountsManager.USERNAME_EXPERIMENT_NAME)))
         .thenReturn(true);
 
+    usernameGenerator = new UsernameGenerator(1, 2, 10);
     accountsManager = new AccountsManager(
         accounts,
         phoneNumberIdentifiers,
@@ -155,7 +157,7 @@ class AccountsManagerUsernameIntegrationTest {
         mock(SecureStorageClient.class),
         mock(SecureBackupClient.class),
         mock(ClientPresenceManager.class),
-        new UsernameGenerator(1, 2, 10),
+        usernameGenerator,
         experimentEnrollmentManager,
         mock(Clock.class));
   }
@@ -197,7 +199,7 @@ class AccountsManagerUsernameIntegrationTest {
           .tableName(USERNAMES_TABLE_NAME)
           .item(Map.of(
               Accounts.KEY_ACCOUNT_UUID, AttributeValues.fromUUID(UUID.randomUUID()),
-              Accounts.ATTR_USERNAME, AttributeValues.fromString(UsernameGenerator.fromParts("n00bkiller", i))))
+              Accounts.ATTR_USERNAME, AttributeValues.fromString(usernameGenerator.fromParts("n00bkiller", i))))
           .build());
     }
     assertThrows(UsernameNotAvailableException.class, () -> accountsManager.setUsername(account, "n00bkiller", null));
@@ -213,7 +215,7 @@ class AccountsManagerUsernameIntegrationTest {
           .tableName(USERNAMES_TABLE_NAME)
           .item(Map.of(
               Accounts.KEY_ACCOUNT_UUID, AttributeValues.fromUUID(UUID.randomUUID()),
-              Accounts.ATTR_USERNAME, AttributeValues.fromString(UsernameGenerator.fromParts("n00bkiller", i))))
+              Accounts.ATTR_USERNAME, AttributeValues.fromString(usernameGenerator.fromParts("n00bkiller", i))))
           .build());
     }
 
diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/UsernameGeneratorTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/UsernameGeneratorTest.java
index c19d40b4d..8ad69d348 100644
--- a/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/UsernameGeneratorTest.java
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/tests/util/UsernameGeneratorTest.java
@@ -5,7 +5,6 @@ import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
-import org.junit.jupiter.params.provider.ValueSource;
 import org.whispersystems.textsecuregcm.storage.UsernameNotAvailableException;
 import org.whispersystems.textsecuregcm.util.UsernameGenerator;
 
@@ -63,6 +62,15 @@ public class UsernameGeneratorTest {
     );
   }
 
+  @Test
+  public void zeroPadDiscriminators() {
+    final UsernameGenerator generator = new UsernameGenerator(4, 5, 1);
+    assertThat(generator.fromParts("test", 1)).isEqualTo("test#0001");
+    assertThat(generator.fromParts("test", 123)).isEqualTo("test#0123");
+    assertThat(generator.fromParts("test", 9999)).isEqualTo("test#9999");
+    assertThat(generator.fromParts("test", 99999)).isEqualTo("test#99999");
+  }
+
   @Test
   public void expectedWidth() throws UsernameNotAvailableException {
     String username = new UsernameGenerator(1, 6, 1).generateAvailableUsername("test", t -> true);