Brian Lee Diemer (born October 10, 1961) is a former American track and field athlete, who mainly competed in the 3000 metre steeplechase during his career.

He graduated from the University of Michigan in 1983 after taking third in the 2 mile at the NCAA Indoor Track and Field Championships, held in nearby Detroit.

He competed for the United States in the 1984 Summer Olympics held in Los Angeles, United States in the 3000 metre steeplechase where he won the bronze medal in a career best time of 8:14.06.

Diemer won four The Athletics Congress (TAC) and U.S. Track and Field (USATF) championships over that time (1988, 1989, 1990 and 1992).

In 1989 he received the Glenn Cunningham Award as the outstanding American male distance runner for the year.