The Master Key System is a personal development book by Charles F. Haanel that was originally published as a 24-week correspondence course. 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. The book describes many 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.

Charles Francis Haanel (1866 – 1949) was an American author, philosopher and a businessman. He is best known for his contributions to the New Thought movement through his book The Master Key System.