Aqua is the graphical user interface, design language and visual theme of Apple's macOS operating system. It was originally based on the theme of water, with droplet-like components and a liberal use of reflection effects and translucency. Its goal is to "incorporate color, depth, translucence, and complex textures into a visually appealing interface" in macOS applications. At its introduction, Steve Jobs noted that "... it's liquid, one of the design goals was when you saw it you wanted to lick it".

Aqua was first introduced at the 2000 Macworld Conference & Expo in San Francisco. Its first appearance in a commercial product was in the July 2000 release of iMovie 2, followed by Mac OS X 10.0 the following year. Aqua is the successor to Platinum, which was used in Mac OS 8, Mac OS 9 and Mac OS X Server 1.2. The appearance of Aqua has changed frequently over the years, most recently and drastically with the release of macOS Big Sur in 2020 which Apple calls the "biggest design upgrade since the introduction of Mac OS X."