Gregor and the Marks of Secret is a high fantasy/epic fantasy novel, the fourth book in the critically acclaimed The Underland Chronicles by Suzanne Collins. It picks up soon after the end of Gregor and the Curse of the Warmbloods.

The Underland Chronicles is a series of five high fantasy novels by Suzanne Collins, first published between 2003 and 2007. It tells the story of a boy named Gregor and his adventures in the "Underland", a subterranean world located under New York City. The Underland is inhabited by humans who traveled below hundreds of years ago, along with various giant versions of creatures like bats, cockroaches, and rats. According to the author, the series involves many topics relating to war, including biological warfare, genocide, and military intelligence. While not as well known as the author's subsequent Hunger Games series, it has been reviewed favorably by many critics.