Robert F. "Ace" Gruenig (March 12, 1913 – August 11, 1958) was an American amateur basketball player during the 1930s and 1940s.
The 6 ft. 8 in. (203 cm) Gruenig is considered as one of the game's first great big men. The Chicago, Illinois native led his high school, Crane Tech, to the Chicago Public High School League championship in 1931. He attended Northwestern University, but withdrew after his freshman year without having played for the varsity. While playing for several AAU teams in the following decade, Gruenig was named AAU All-America 10 times (1937–40, 1942–46, 1948). Furthermore, he led the Denver Safeway (1937), Denver Nuggets (1939) and Denver American Legion (1942) teams to AAU championships. On August 11, 1963, Gruenig was enshrined to the Basketball Hall of Fame.