Windows 1.0 is the first major release of Microsoft Windows, a family of graphical operating systems for personal computers developed by Microsoft. It was first released to manufacturing in the United States on November 20, 1985, while the European version was released as Windows 1.02 in May 1986.

Its development began after the Microsoft co-founder and spearhead of Windows 1.0, Bill Gates, saw a demonstration of a similar software suite, Visi On, at COMDEX in 1982. The operating environment was showcased to the public in November 1983, although it ended up being released two years later. Windows 1.0 runs on MS-DOS, as a 16-bit shell program known as MS-DOS Executive, and it provides an environment which can run graphical programs designed for Windows, as well as existing MS-DOS software. It introduced multitasking and the use of the mouse, and various built-in programs such as Calculator, Paint, and Notepad. The operating environment does not allow its windows to overlap, and instead, the windows are tiled. Windows 1.0 received four releases numbered 1.01 through 1.04, mainly adding support for newer hardware or additional languages.

The system received lukewarm reviews; critics raised concerns about not fulfilling expectations, its compatibility with very little software, and its performance issues, while it has also received positive responses to Microsoft's early presentations and support from a number of hardware- and software-makers. Its last release was 1.04, and it was succeeded by Windows 2.0, which was released in December 1987. Microsoft ended its support for Windows 1.0 on December 31, 2001, making it the longest-supported out of all versions of Windows.