Which company developed the Windows operating system?

Microsoft developed the Windows operating system.

Microsoft announced Windows in 1983 and released Windows 1.0 on November 20, 1985. The first version was not a completely independent operating system; it was a graphical shell that ran on top of MS-DOS. It gave users visual windows, menus, icons, and mouse-based controls instead of requiring every task to be typed at a command prompt.

Windows 3.0, released in 1990, brought Microsoft major commercial success. Windows 95 later introduced familiar features such as the Start menu, taskbar, and Windows Explorer. The platform eventually evolved into the Windows NT family, which forms the technical foundation of modern Windows editions.

A common mix-up is confusing Microsoft with IBM. IBM partnered with Microsoft on OS/2, but Microsoft created and developed Windows. Apple developed macOS, while Google develops Android and ChromeOS.