Windows 3.1x (codenamed Janus) is a series of 16-bit operating systems produced by Microsoft for use on personal computers. The series began with Windows 3.1, which was first sold during April 1992 as a successor to Windows 3.0. Subsequent versions were released between 1992 and 1994 until the series was superseded by Windows 95. During its lifespan, Windows 3.1 introduced several enhancements to the still MS-DOS-based platform, including improved system stability, expanded support for multimedia, TrueType fonts, and workgroup networking.

Windows 3.1 was originally released on April 6, 1992; official support for Windows 3.1 ended on December 31, 2001, and OEM licensing for Windows for Workgroups 3.11 on embedded systems continued to be available until November 1, 2008.