The Age of Reason, Being an Investigation of True and Fabulous Theology is a work by English and American political activist Thomas Paine, arguing for the philosophical position of deism. It follows in the tradition of 18th-century British deism, and challenges institutionalized religion and the legitimacy of the Bible. It was published in three parts in 1794, 1795, and 1807.

In Part I, Paine outlines his major arguments and personal creed. In Parts II and III he analyzes specific portions of the Bible in order to demonstrate that it is not the revealed word of God. Most of Paine's arguments had long been available to the educated elite, but by presenting them in an engaging and irreverent style, he made deism appealing and accessible to a mass audience.