diff --git a/tests/phpunit/tests/functions/numberFormatI18n.php b/tests/phpunit/tests/functions/numberFormatI18n.php
new file mode 100644
index 0000000000..8c514b319c
--- /dev/null
+++ b/tests/phpunit/tests/functions/numberFormatI18n.php
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * Tests for number_format_i18n()
+ *
+ * @group functions.php
+ * @group i18n
+ */
+class Tests_Functions_Number_Format_I18n extends WP_UnitTestCase {
+	public function test_should_fall_back_to_number_format_when_wp_locale_is_not_set() {
+		$locale               = clone $GLOBALS['wp_locale'];
+		$GLOBALS['wp_locale'] = null;
+
+		$actual_1 = number_format_i18n( 123456.789, 0 );
+		$actual_2 = number_format_i18n( 123456.789, 4 );
+
+		$GLOBALS['wp_locale'] = $locale;
+
+		$this->assertEquals( '123,457', $actual_1 );
+		$this->assertEquals( '123,456.7890', $actual_2 );
+	}
+
+	public function test_should_respect_number_format_of_locale() {
+		$decimal_point = $GLOBALS['wp_locale']->number_format['decimal_point'];
+		$thousands_sep = $GLOBALS['wp_locale']->number_format['thousands_sep'];
+
+		$GLOBALS['wp_locale']->number_format['decimal_point'] = '@';
+		$GLOBALS['wp_locale']->number_format['thousands_sep'] = '^';
+
+		$actual_1 = number_format_i18n( 123456.789, 0 );
+		$actual_2 = number_format_i18n( 123456.789, 4 );
+
+		$GLOBALS['wp_locale']->number_format['decimal_point'] = $decimal_point;
+		$GLOBALS['wp_locale']->number_format['thousands_sep'] = $thousands_sep;
+
+		$this->assertEquals( '123^457', $actual_1 );
+		$this->assertEquals( '123^456@7890', $actual_2 );
+	}
+
+	public function test_should_default_to_en_us_format() {
+		$this->assertEquals( '123,457', number_format_i18n( 123456.789, 0 ) );
+		$this->assertEquals( '123,456.7890', number_format_i18n( 123456.789, 4 ) );
+	}
+
+	public function test_should_handle_negative_precision() {
+		$this->assertEquals( '123,457', number_format_i18n( 123456.789, 0 ) );
+		$this->assertEquals( '123,456.7890', number_format_i18n( 123456.789, -4 ) );
+	}
+}