diff --git a/src/css.js b/src/css.js
index c17265ced..7a225d9a9 100644
--- a/src/css.js
+++ b/src/css.js
@@ -28,6 +28,7 @@ var
 	// except "table", "table-cell", or "table-caption"
 	// See here for display values: https://developer.mozilla.org/en-US/docs/CSS/display
 	rdisplayswap = /^(none|table(?!-c[ea]).+)/,
+	rcustomProp = /^--/,
 	cssShow = { position: "absolute", visibility: "hidden", display: "block" },
 	cssNormalTransform = {
 		letterSpacing: "0",
@@ -57,6 +58,16 @@ function vendorPropName( name ) {
 	}
 }
 
+// Return a property mapped along what jQuery.cssProps suggests or to
+// a vendor prefixed property.
+function finalPropName( name ) {
+	var ret = jQuery.cssProps[ name ];
+	if ( !ret ) {
+		ret = jQuery.cssProps[ name ] = vendorPropName( name ) || name;
+	}
+	return ret;
+}
+
 function setPositiveNumber( elem, value, subtract ) {
 
 	// Any relative (+/-) values have already been
@@ -218,10 +229,15 @@ jQuery.extend( {
 		// Make sure that we're working with the right name
 		var ret, type, hooks,
 			origName = jQuery.camelCase( name ),
+			isCustomProp = rcustomProp.test( name ),
 			style = elem.style;
 
-		name = jQuery.cssProps[ origName ] ||
-			( jQuery.cssProps[ origName ] = vendorPropName( origName ) || origName );
+		// Make sure that we're working with the right name. We don't
+		// want to query the value if it is a CSS custom property
+		// since they are user-defined.
+		if ( !isCustomProp ) {
+			name = finalPropName( origName );
+		}
 
 		// Gets hook for the prefixed version, then unprefixed version
 		hooks = jQuery.cssHooks[ name ] || jQuery.cssHooks[ origName ];
@@ -257,7 +273,11 @@ jQuery.extend( {
 			if ( !hooks || !( "set" in hooks ) ||
 				( value = hooks.set( elem, value, extra ) ) !== undefined ) {
 
-				style[ name ] = value;
+				if ( isCustomProp ) {
+					style.setProperty( name, value );
+				} else {
+					style[ name ] = value;
+				}
 			}
 
 		} else {
@@ -276,11 +296,15 @@ jQuery.extend( {
 
 	css: function( elem, name, extra, styles ) {
 		var val, num, hooks,
-			origName = jQuery.camelCase( name );
+			origName = jQuery.camelCase( name ),
+			isCustomProp = rcustomProp.test( name );
 
-		// Make sure that we're working with the right name
-		name = jQuery.cssProps[ origName ] ||
-			( jQuery.cssProps[ origName ] = vendorPropName( origName ) || origName );
+		// Make sure that we're working with the right name. We don't
+		// want to modify the value if it is a CSS custom property
+		// since they are user-defined.
+		if ( !isCustomProp ) {
+			name = finalPropName( origName );
+		}
 
 		// Try prefixed name followed by the unprefixed name
 		hooks = jQuery.cssHooks[ name ] || jQuery.cssHooks[ origName ];
@@ -305,6 +329,7 @@ jQuery.extend( {
 			num = parseFloat( val );
 			return extra === true || isFinite( num ) ? num || 0 : val;
 		}
+
 		return val;
 	}
 } );
diff --git a/src/css/curCSS.js b/src/css/curCSS.js
index 313da4222..de5333030 100644
--- a/src/css/curCSS.js
+++ b/src/css/curCSS.js
@@ -15,8 +15,9 @@ function curCSS( elem, name, computed ) {
 
 	computed = computed || getStyles( elem );
 
-	// Support: IE <=9 only
-	// getPropertyValue is only needed for .css('filter') (#12537)
+	// getPropertyValue is needed for:
+	//   .css('filter') (IE 9 only, #12537)
+	//   .css('--customProperty) (#3144)
 	if ( computed ) {
 		ret = computed.getPropertyValue( name ) || computed[ name ];
 
diff --git a/test/unit/css.js b/test/unit/css.js
index 2f529b691..460023b62 100644
--- a/test/unit/css.js
+++ b/test/unit/css.js
@@ -1556,4 +1556,68 @@ QUnit.test( "Do not throw on frame elements from css method (#15098)", function(
 
 } )();
 
+
+QUnit.test( "css(--customProperty)", function( assert ) {
+	jQuery( "#qunit-fixture" ).append(
+		"<style>\n" +
+		"    .test__customProperties {\n" +
+		"        --prop1:val1;\n" +
+		"        --prop2: val2;\n" +
+		"        --prop3:val3 ;\n" +
+		"        --prop4:\"val4\";\n" +
+		"        --prop5:'val5';\n" +
+		"    }\n" +
+		"</style>"
+	);
+
+	var div = jQuery( "<div>" ).appendTo( "#qunit-fixture" ),
+		$elem = jQuery( "<div>" ).addClass( "test__customProperties" ).appendTo( "#qunit-fixture" ),
+		webkit = /\bsafari\b/i.test( navigator.userAgent ) &&
+			!/\firefox\b/i.test( navigator.userAgent ) &&
+			!/\edge\b/i.test( navigator.userAgent ),
+		oldSafari = webkit && ( /\b9\.\d(\.\d+)* safari/i.test( navigator.userAgent ) ||
+			/\b10\.0(\.\d+)* safari/i.test( navigator.userAgent ) ),
+		expected = 10;
+
+	if ( webkit ) {
+		expected -= 2;
+	}
+	if ( oldSafari ) {
+		expected -= 2;
+	}
+	assert.expect( expected );
+
+	div.css( "--color", "blue" );
+	assert.equal( div.css( "--color" ), "blue", "Modified CSS custom property using string" );
+
+	div.css( "--color", "yellow" );
+	assert.equal( div.css( "--color" ), "yellow", "Overwrite CSS custom property" );
+
+	div.css( { "--color": "red" } );
+	assert.equal( div.css( "--color" ), "red", "Modified CSS custom property using object" );
+
+	div.css( { "--mixedCase": "green" } );
+	assert.equal( div.css( "--mixedCase" ), "green", "Modified CSS custom property with mixed case" );
+
+	div.css( { "--theme-dark": "purple" } );
+	assert.equal( div.css( "--theme-dark" ), "purple", "Modified CSS custom property with dashed name" );
+
+	assert.equal( $elem.css( "--prop1" ), "val1", "Basic CSS custom property" );
+
+	// Support: Safari 9.1-10.0 only
+	// Safari collapses whitespaces & quotes. Ignore it.
+	if ( !oldSafari ) {
+		assert.equal( $elem.css( "--prop2" ), " val2", "Preceding whitespace maintained" );
+		assert.equal( $elem.css( "--prop3" ), "val3 ", "Following whitespace maintained" );
+	}
+
+	// Support: Chrome 49-55, Safari 9.1-10.0
+	// Chrome treats single quotes as double ones.
+	// Safari treats double quotes as single ones.
+	if ( !webkit ) {
+		assert.equal( $elem.css( "--prop4" ), "\"val4\"", "Works with double quotes" );
+		assert.equal( $elem.css( "--prop5" ), "'val5'", "Works with single quotes" );
+	}
+} );
+
 }