The Law of the 45's (also known as The Mysterious Mr. Sheffield in the United Kingdom) is a 1935 American Western film directed by John P. McCarthy. The screenplay was based on the 1933 novel of the same name by William Colt MacDonald. It was the first film to be made of MacDonald's characters The Three Mesquiteers, that later became a film series at Republic Pictures. Though only two of the characters, Tucson and Stoney, appeared in this film, Williams would appear as the missing member "Lullaby" Joslyn in Powdersmoke Range shot in the same year for RKO.

Previously the Alexander brothers Arthur Alexander and Max Alexander had released a series of Westerns starring Guinn Williams under their Beacon Pictures company. Law of the 45s was made by Max's Normandy Pictures.