The John D. and Catherine T. MacArthur Foundation is a private foundation that makes grants and impact investments to support non-profit organizations in approximately 117 countries around the world. It has an endowment of $7.6 billion and provides approximately $260 million annually in grants and impact investments.It is based in Chicago, and in 2014 it was the 12th-largest private foundation in the United States.It has awarded more than US$8.27 billion since its first grants in 1978.

The foundation's stated purpose is to support "creative people, effective institutions, and influential networks building a more just, verdant, and peaceful world".MacArthur's grant-making priorities include mitigating climate change, reducing jail populations, decreasing nuclear threats, supporting nonprofit journalism, and funding local needs in its hometown of Chicago.