Authorities say it happened in the area of Calumet Ranch Trail and Knaust Rd.
A witness told police he saw the suspect, in his vehicle with no clothes on the Spencer Creek Library parking lot.
Lieutenant John Romas told KMOX his office is currently investigating five incidents, all within the same area on Olive and New Ballas.
An O’Fallon, Illinois man now faces a Felony-Public Indecency charge after exposing himself in public Saturday for the third time.
UMSL police say man exposed himself to a female student Thursday around noon in a 2nd floor hallway of Clark Hall.