Windows 9x是以Windows 95内核作为蓝本的微软操作系统的通称。这包括了Windows 95、Windows 98的所有版本，以及2000年发布的Windows Me。

Windows 9x所使用的内部发行版本号为“4.x”，而之前基于MS-DOS的Windows版本则使用等于或小于3.9的版本号。Windows NT使用类似但独立的、介于3.5与4.0的版本号。Windows 9x的第一继承人Windows XP使用的版本号为5.1。

Windows 9x闻名于它在桌面的使用；在1998年，Windows占有了操作系统市场份额的82%。

Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced from 1995 to 2000, which were based on the Windows 95 kernel and its underlying foundation of MS-DOS, both of which were updated in subsequent versions. This includes all versions of Windows 95 and Windows 98. Windows ME is sometimes included.