Windows 2000 is a business-oriented operating system that was produced by Microsoft in the United States and was released as part of the Windows NT family of operating systems. It was succeeded by Windows XP in 2001, releasing to manufacturing on December 15, 1999 and being officially released to retail on February 17, 2000. It is the successor to Windows NT 4.0.

Four editions of Windows 2000 were released: Professional, Server, Advanced Server, and Datacenter Server; the latter was both released to manufacturing and launched months after the other editions. While each edition of Windows 2000 was targeted at a different market, they shared a core set of features, including many system utilities such as the Microsoft Management Console and standard system administration applications.

Windows 2000 introduces NTFS 3.0, Encrypting File System, as well as basic and dynamic disk storage. Support for people with disabilities was improved over Windows NT 4.0 with a number of new assistive technologies, and Microsoft increased support for different languages and locale information. The Windows 2000 Server family has additional features, most notably the introduction of Active Directory, which in the years following became a widely used directory service in business environments.

Microsoft marketed Windows 2000 as the most secure Windows version ever at the time; however, it became the target of a number of high-profile virus attacks such as Code Red and Nimda. For ten years after its release, it continued to receive patches for security vulnerabilities nearly every month until reaching the end of its lifecycle on July 13, 2010.

Windows 2000 is the last version of Microsoft Windows to display the "Windows NT" designation, and the last version where the desktop and server versions of Windows shared the same name. It is succeeded by Windows XP (released in October 2001) and Windows Server 2003 (released in April 2003).