One Spy Too Many, starring Robert Vaughn and David McCallum, is the 1966 feature-length film version of The Man from U.N.C.L.E.'s two-part season two premiere "Alexander the Greater Affair". It is the third such feature film that used as its basis a reedited version of one or more episodes from the series. In this instance, the film took the two-part episode and added in a subplot featuring Yvonne Craig as an U.N.C.L.E. operative carrying on a flirtatious relationship with Napoleon Solo (Robert Vaughn); Craig does not appear in the television episodes. Both episodes were written by Dean Hargrove and directed by Joseph Sargent.

It also added in and substituted scenes that, while not out of place in a 1960s U.S. spy film, were more explicitly sexual than generally shown on U.S. television at the time. Whereas the earlier U.N.C.L.E. films added material to a single episode to create a feature-length movie, "One Spy Too Many" removed certain elements of the two-part episode (e.g., scenes with Alexander's parents) to allow for the added subplot with Craig and other enhanced scenes within the film's overall running time. This was the last film culled from the series to be theatrically released in the U.S. (in late 1966).