diff --git a/src/wp-admin/network/user-new.php b/src/wp-admin/network/user-new.php
index 52e4d9fa7e..4b330c0819 100644
--- a/src/wp-admin/network/user-new.php
+++ b/src/wp-admin/network/user-new.php
@@ -89,7 +89,7 @@ if ( isset( $add_user_errors ) && is_wp_error( $add_user_errors ) ) { ?>
 	<table class="form-table">
 		<tr class="form-field form-required">
 			<th scope="row"><label for="username"><?php _e( 'Username' ) ?></label></th>
-			<td><input type="text" class="regular-text" name="user[username]" id="username" autocapitalize="none" autocorrect="off" /></td>
+			<td><input type="text" class="regular-text" name="user[username]" id="username" autocapitalize="none" autocorrect="off" maxlength="60" /></td>
 		</tr>
 		<tr class="form-field form-required">
 			<th scope="row"><label for="email"><?php _e( 'Email' ) ?></label></th>
diff --git a/src/wp-admin/user-new.php b/src/wp-admin/user-new.php
index 67da518f6e..83cb90e0d1 100644
--- a/src/wp-admin/user-new.php
+++ b/src/wp-admin/user-new.php
@@ -375,7 +375,7 @@ $new_user_ignore_pass = $creating && isset( $_POST['noconfirmation'] ) ? wp_unsl
 <table class="form-table">
 	<tr class="form-field form-required">
 		<th scope="row"><label for="user_login"><?php _e('Username'); ?> <span class="description"><?php _e('(required)'); ?></span></label></th>
-		<td><input name="user_login" type="text" id="user_login" value="<?php echo esc_attr( $new_user_login ); ?>" aria-required="true" autocapitalize="none" autocorrect="off" /></td>
+		<td><input name="user_login" type="text" id="user_login" value="<?php echo esc_attr( $new_user_login ); ?>" aria-required="true" autocapitalize="none" autocorrect="off" maxlength="60" /></td>
 	</tr>
 	<tr class="form-field form-required">
 		<th scope="row"><label for="email"><?php _e('Email'); ?> <span class="description"><?php _e('(required)'); ?></span></label></th>
diff --git a/src/wp-includes/user-functions.php b/src/wp-includes/user-functions.php
index e1b933e6cf..19f564344d 100644
--- a/src/wp-includes/user-functions.php
+++ b/src/wp-includes/user-functions.php
@@ -1245,19 +1245,28 @@ function wp_insert_user( $userdata ) {
 	//Remove any non-printable chars from the login string to see if we have ended up with an empty username
 	$user_login = trim( $pre_user_login );
 
+	// user_login must be between 0 and 60 characters.
 	if ( empty( $user_login ) ) {
 		return new WP_Error('empty_user_login', __('Cannot create a user with an empty login name.') );
+	} elseif ( mb_strlen( $user_login ) > 60 ) {
+		return new WP_Error( 'user_login_too_long', __( 'Username may not be longer than 60 characters.' ) );
 	}
+
 	if ( ! $update && username_exists( $user_login ) ) {
 		return new WP_Error( 'existing_user_login', __( 'Sorry, that username already exists!' ) );
 	}
 
-	// If a nicename is provided, remove unsafe user characters before
-	// using it. Otherwise build a nicename from the user_login.
+	/*
+	 * If a nicename is provided, remove unsafe user characters before using it.
+	 * Otherwise build a nicename from the user_login.
+	 */
 	if ( ! empty( $userdata['user_nicename'] ) ) {
 		$user_nicename = sanitize_user( $userdata['user_nicename'], true );
+		if ( mb_strlen( $user_nicename ) > 50 ) {
+			return new WP_Error( 'user_nicename_too_long', __( 'Nicename may not be longer than 50 characters.' ) );
+		}
 	} else {
-		$user_nicename = $user_login;
+		$user_nicename = mb_substr( $user_login, 0, 50 );
 	}
 
 	$user_nicename = sanitize_title( $user_nicename );
@@ -1395,7 +1404,9 @@ function wp_insert_user( $userdata ) {
 	if ( $user_nicename_check ) {
 		$suffix = 2;
 		while ($user_nicename_check) {
-			$alt_user_nicename = $user_nicename . "-$suffix";
+			// user_nicename allows 50 chars. Subtract one for a hyphen, plus the length of the suffix.
+			$base_length = 49 - mb_strlen( $suffix );
+			$alt_user_nicename = mb_substr( $user_nicename, 0, $base_length ) . "-$suffix";
 			$user_nicename_check = $wpdb->get_var( $wpdb->prepare("SELECT ID FROM $wpdb->users WHERE user_nicename = %s AND user_login != %s LIMIT 1" , $alt_user_nicename, $user_login));
 			$suffix++;
 		}
diff --git a/tests/phpunit/tests/user.php b/tests/phpunit/tests/user.php
index 4dc281cb44..9a3b9ab017 100644
--- a/tests/phpunit/tests/user.php
+++ b/tests/phpunit/tests/user.php
@@ -574,6 +574,97 @@ class Tests_User extends WP_UnitTestCase {
 		$this->assertSame( $user->user_nicename, $updated_user->user_nicename );
 	}
 
+	/**
+	 * @ticket 33793
+	 */
+	public function test_wp_insert_user_should_reject_user_login_over_60_characters() {
+		$user_login = str_repeat( 'a', 61 );
+		$u = wp_insert_user( array(
+			'user_login' => $user_login,
+			'user_email' => $user_login . '@example.com',
+			'user_pass' => 'password',
+			'user_nicename' => 'something-short',
+		) );
+
+		$this->assertWPError( $u );
+		$this->assertSame( 'user_login_too_long', $u->get_error_code() );
+	}
+
+	/**
+	 * @ticket 33793
+	 */
+	public function test_wp_insert_user_should_reject_user_nicename_over_50_characters() {
+		$user_nicename = str_repeat( 'a', 51 );
+		$u = wp_insert_user( array(
+			'user_login' => 'mynicenamehas50chars',
+			'user_email' => $user_nicename . '@example.com',
+			'user_pass' => 'password',
+			'user_nicename' => $user_nicename,
+		) );
+
+		$this->assertWPError( $u );
+		$this->assertSame( 'user_nicename_too_long', $u->get_error_code() );
+	}
+
+	/**
+	 * @ticket 33793
+	 */
+	public function test_wp_insert_user_should_not_generate_user_nicename_longer_than_50_chars() {
+		$user_login = str_repeat( 'a', 55 );
+		$u = wp_insert_user( array(
+			'user_login' => $user_login,
+			'user_email' => $user_login . '@example.com',
+			'user_pass' => 'password',
+		) );
+
+		$this->assertNotEmpty( $u );
+		$user = new WP_User( $u );
+		$expected = str_repeat( 'a', 50 );
+		$this->assertSame( $expected, $user->user_nicename );
+	}
+
+	/**
+	 * @ticket 33793
+	 */
+	public function test_wp_insert_user_should_not_truncate_to_a_duplicate_user_nicename() {
+		$u1 = $this->factory->user->create( array(
+			'user_nicename' => str_repeat( 'a', 50 ),
+		) );
+
+		$user_login = str_repeat( 'a', 55 );
+		$u = wp_insert_user( array(
+			'user_login' => $user_login,
+			'user_email' => $user_login . '@example.com',
+			'user_pass' => 'password',
+		) );
+
+		$this->assertNotEmpty( $u );
+		$user = new WP_User( $u );
+		$expected = str_repeat( 'a', 48 ) . '-2';
+		$this->assertSame( $expected, $user->user_nicename );
+	}
+
+	/**
+	 * @ticket 33793
+	 */
+	public function test_wp_insert_user_should_not_truncate_to_a_duplicate_user_nicename_when_suffix_has_more_than_one_character() {
+		$users = $this->factory->user->create_many( 9, array(
+			'user_nicename' => str_repeat( 'a', 50 ),
+		) );
+
+		$user_login = str_repeat( 'a', 55 );
+		$u = wp_insert_user( array(
+			'user_login' => $user_login,
+			'user_email' => $user_login . '@example.com',
+			'user_pass' => 'password',
+		) );
+
+		$this->assertNotEmpty( $u );
+		$user = new WP_User( $u );
+		$expected = str_repeat( 'a', 47 ) . '-10';
+		$this->assertSame( $expected, $user->user_nicename );
+	}
+
 	function test_changing_email_invalidates_password_reset_key() {
 		global $wpdb;