The Master Key System is a personal development book by Charles F. Haanel (1866–1949) that was originally published as a 24-week correspondence course in 1912, and then in book form in 1916. The ideas it describes and explains come mostly from New Thought philosophy. It was one of the main sources of inspiration for Rhonda Byrne’s film and book The Secret (2006).

The book describes many New Thought beliefs such as the law of attraction, creative visualization and man’s unity with God, and teaches the importance of truth, harmonious thinking and the ability to concentrate. Each of the 24 chapters contains an introduction, followed by a sequentially numbered section which includes an exercise towards the end, followed by a section with questions and answers.