Clark S. Gruening ( // GREEN-ing; born March 28, 1943)  is an attorney and Democratic Party politician from the U.S. state of Alaska. He is chiefly known as the second of three persons to defeat the incumbent holder of Alaska's Class 3 United States Senate seat in the primary election.
Clark Gruening was born in San Francisco, California, the son of Huntington Sanders "Hunt" Gruening, the second-born and (after 1955) only surviving son of Ernest Gruening, who at the time of Clark's birth was governor of the Territory of Alaska. He grew up in Juneau, Alaska, where his father was an airline pilot and executive.  Clark graduated from Juneau-Douglas High School in 1961. He received a B.A. in political science from the University of Oregon in 1965 and a J.D. from George Washington University in 1969. He moved to Anchorage that same year. 
Gruening, an attorney outside politics, was elected to two terms in the Alaska House of Representatives starting in 1974.  In the 1980 U.S. Senate election in Alaska, he ran in the Democratic Senate primary, defeating incumbent Senator Mike Gravel.  Gravel had himself defeated Ernest Gruening in the Democratic primary twelve years earlier. 
Both election results were felt to have been attributed to the blanket primary system used in Alaska, which was brand new in 1968 and eventually discontinued due to complaints by individual political parties that members of other parties had a hand in the selection of their party's nominees. Specifically, it was felt that Republicans and supporters of Frank Murkowski voted in large numbers for Gruening, in the hopes of eliminating Gravel. Gravel, extremely unpopular with Alaskan voters at the time, nonetheless was felt to have posed a more serious challenge to Murkowski largely on account of his incumbency.
Gruening went on to lose the 1980 general election to Republican nominee Murkowski. 
His family consisted of his brothers Johnathon, who died and was adopted at a young age, Bradford "Brad" and his twin Winthrop "Win", and Peter. Peter's children are Parker and Landon Gruening, with stepdaughter Erika Tingley. His sisters are Kimberley, whose children are Hunter and Colby McNickle, and Tiffany, whose children are McKinnon and Carsyn Williams.
- State of Alaska Official Election Pamphlet (Senate Districts A-C and House Districts 1-4 ed.). Juneau: Office of the Alaska Lieutenant Governor. 1980. p. 20.
- Naske, Claus-M. (2004). Ernest Gruening : Alaska's Greatest Governor. Fairbanks: University of Alaska Press. p. 182. ISBN 1-889963-34-8.
- Clark Gruening at 100 Years of Alaska's Legislature