ICNS is an icon file format used by Mac OS operating system. ICNS files can store icons of the following dimensions (in pixels): 16×16, 32×32, 48×48, 128×128, 256×256, 512×512, and 1024×1024. Operating system can automatically adjust icon size.

Icon images stored in ICNS files support 8-bit alpha channel. Icon of the same theme can use different settings depending on context (for example open and closed folder). ICNS format underwent many changes and with time became sophisticated enough to serve as base for other similar formats.