A NUPROJ file is a project created by Visual Studio with the NuBuild Project System extension installed. It contains a project, which includes build configuration properties. NUPROJ files are used to build .NUPKG NuGet packages from .NUSPEC files and manage them.

NUPROJ files are saved in the same format as MSBuild projects, which is a simple, XML format. The format allows developers to describe the items that need to be built and how they need to be built with various configurations and platforms. With the NuBuild Project System extension installed with Visual Studio, you can build NuGet packages in Visual Studio using the NUPROJ file. You must also have the NuGet package manager installed.