Charles Donald "Sam" Gemar (born August 4, 1955) is an American former astronaut with NASA and a lieutenant colonel in the United States Army.

In November 1973, he was assigned to the XVIII Airborne Corps at Fort Bragg, North Carolina, where he received an appointment to the U.S. Military Academy Preparatory School at Fort Belvoir, Virginia, and later a Department of the Army appointment to join the United States Military Academy Class of 1979.

[1] Selected by NASA in June 1985, Gemar completed a one-year training and evaluation program and became an astronaut in July 1986.

On his first mission, Gemar served on the five-man crew of STS-38 which launched at night from the Kennedy Space Center, Florida, on November 15, 1990.

Sixty experiments or investigations were conducted in many scientific and engineering disciplines including: materials science, human physiology, biotechnology, protein crystal growth, robotics, structural dynamics, atmospheric ozone monitoring and spacecraft glow.

During the spacecraft glow investigation, Columbia's orbital altitude was lowered to 105 nautical miles (194 km), the lowest ever flown by a Space Shuttle.