Mono is a free and open-source project to create an Ecma standard-compliant .NET Framework-compatible software framework, including a C# compiler and a Common Language Runtime. Originally by Ximian, it was later acquired by Novell, and is now being led by Xamarin, a subsidiary of Microsoft and the .NET Foundation. The stated purpose of Mono is not only to be able to run Microsoft .NET applications cross-platform but also to bring better development tools to Linux developers. Mono can be run on many software systems including Android, most Linux distributions, BSD, macOS, Windows, Solaris, and even some game consoles such as PlayStation 3, Wii, and Xbox 360.

The Mono project has been controversial within the open-source community, as it implements portions of .NET Framework that may be covered by Microsoft patents. Although standardized portions of .NET Framework are covered under Microsoft Open Specification Promise—a covenant stating that Microsoft will not assert its patents against implementations of its specifications under certain conditions—other portions are not, which led to concerns that the Mono project could become the target of patent infringement lawsuits. Following Microsoft's open-sourcing of several core .NET technologies since 2014 and its acquisition of Xamarin at the beginning of 2016, an updated patent promise has been issued for the Mono project.

The logo of Mono is a stylized monkey's face, mono being Spanish for monkey.