Ian Mackinnon Caldwell (born March 18, 1976) is an American novelist known for co-authoring the 2004 novel The Rule of Four. His second book, The Fifth Gospel, was published in 2015.

After graduating from Princeton University in 1998, Caldwell and his childhood friend Dustin Thomason co-wrote the semi-autobiographical The Rule of Four. Caldwell and Thomason graduated from the Thomas Jefferson High School for Science and Technology, in Fairfax County, Virginia, in 1994. Caldwell was a Phi Beta Kappa in history at Princeton.

In 2005, Caldwell's wife, Meredith, gave birth to their first child, Ethan Sawyer Caldwell. They live in Vienna, Virginia.