diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java
index f49e23f7b..6eedc1811 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/controllers/AccountController.java
@@ -91,6 +91,8 @@ import org.whispersystems.textsecuregcm.util.ForwardedIpUtil;
 import org.whispersystems.textsecuregcm.util.Hex;
 import org.whispersystems.textsecuregcm.util.ImpossiblePhoneNumberException;
 import org.whispersystems.textsecuregcm.util.NonNormalizedPhoneNumberException;
+import org.whispersystems.textsecuregcm.util.Username;
+import org.whispersystems.textsecuregcm.util.UsernameValidator;
 import org.whispersystems.textsecuregcm.util.Util;
 import org.whispersystems.textsecuregcm.util.VerificationCode;
 
@@ -617,22 +619,12 @@ public class AccountController {
   @PUT
   @Path("/username/{username}")
   @Produces(MediaType.APPLICATION_JSON)
-  public Response setUsername(@Auth AuthenticatedAccount auth, @PathParam("username") String username)
+  public Response setUsername(@Auth AuthenticatedAccount auth, @PathParam("username") @Username String username)
       throws RateLimitExceededException {
     rateLimiters.getUsernameSetLimiter().validate(auth.getAccount().getUuid());
 
-    if (username == null || username.isEmpty()) {
-      return Response.status(Response.Status.BAD_REQUEST).build();
-    }
-
-    username = username.toLowerCase();
-
-    if (!username.matches("^[a-z_][a-z0-9_]+$")) {
-      return Response.status(Response.Status.BAD_REQUEST).build();
-    }
-
     try {
-      accounts.setUsername(auth.getAccount(), username);
+      accounts.setUsername(auth.getAccount(), UsernameValidator.getCanonicalUsername(username));
     } catch (final UsernameNotAvailableException e) {
       return Response.status(Response.Status.CONFLICT).build();
     }
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/util/Username.java b/service/src/main/java/org/whispersystems/textsecuregcm/util/Username.java
new file mode 100644
index 000000000..b9d7af788
--- /dev/null
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/util/Username.java
@@ -0,0 +1,27 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.ElementType.PARAMETER;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+import javax.validation.Constraint;
+import javax.validation.Payload;
+
+@Target({ FIELD, PARAMETER })
+@Retention(RUNTIME)
+@Constraint(validatedBy = UsernameValidator.class)
+public @interface Username {
+
+  String message() default "{org.whispersystems.textsecuregcm.util.Username.message}";
+
+  Class<?>[] groups() default { };
+
+  Class<? extends Payload>[] payload() default { };
+}
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameValidator.java b/service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameValidator.java
new file mode 100644
index 000000000..89cd2112d
--- /dev/null
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/util/UsernameValidator.java
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+import org.apache.commons.lang3.StringUtils;
+import javax.validation.ConstraintValidator;
+import javax.validation.ConstraintValidatorContext;
+import java.util.regex.Pattern;
+
+public class UsernameValidator implements ConstraintValidator<Username, String> {
+
+  private static final Pattern USERNAME_PATTERN =
+      Pattern.compile("^[a-z_][a-z0-9_]{3,25}$", Pattern.CASE_INSENSITIVE);
+
+  @Override
+  public boolean isValid(final String username, final ConstraintValidatorContext context) {
+    return StringUtils.isNotBlank(username) && USERNAME_PATTERN.matcher(getCanonicalUsername(username)).matches();
+  }
+
+  public static String getCanonicalUsername(final String username) {
+    return username != null ? username.toLowerCase() : null;
+  }
+}
diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/util/UsernameValidatorTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/util/UsernameValidatorTest.java
new file mode 100644
index 000000000..32e7c3d83
--- /dev/null
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/util/UsernameValidatorTest.java
@@ -0,0 +1,56 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class UsernameValidatorTest {
+
+  @ParameterizedTest
+  @MethodSource
+  void isValid(final String username, final boolean expectValid) {
+    final UsernameValidator usernameValidator = new UsernameValidator();
+
+    assertEquals(expectValid, usernameValidator.isValid(username, null));
+  }
+
+  private static Stream<Arguments> isValid() {
+    return Stream.of(
+        Arguments.of("test", true),
+        Arguments.of("_test", true),
+        Arguments.of("test123", true),
+        Arguments.of("a", false), // Too short
+        Arguments.of("thisIsAReallyReallyReallyLongUsernameThatWeWouldNotAllow", false),
+        Arguments.of("Illegal character", false),
+        Arguments.of("0test", false), // Illegal first character
+        Arguments.of("pаypal", false), // Unicode confusable characters
+        Arguments.of("test\uD83D\uDC4E", false), // Emoji
+        Arguments.of(" ", false),
+        Arguments.of("", false),
+        Arguments.of(null, false)
+    );
+  }
+
+  @ParameterizedTest
+  @MethodSource
+  void getCanonicalUsername(final String username, final String expectedCanonicalUsername) {
+    assertEquals(expectedCanonicalUsername, UsernameValidator.getCanonicalUsername(username));
+  }
+
+  private static Stream<Arguments> getCanonicalUsername() {
+    return Stream.of(
+        Arguments.of("test", "test"),
+        Arguments.of("TEst", "test"),
+        Arguments.of("t_e_S_T", "t_e_s_t"),
+        Arguments.of(null, null)
+    );
+  }
+}