Cassius Marcellus Coolidge (1844-1934) was a United States painter best known for his pictures of anthropomorphized dogs.

Born in upstate New York to abolitionist Quaker farmers, Coolidge was known to friends and family as "Cash." While he had no formal training as an artist his natural aptitude for drawing lead him to create cartoons for his local newspaper when in his twenties. He is credited with creating Comic Foregrounds, life-size cutouts into which one's head was placed so as to be photographed as an amusing character.

In 1903, Coolidge contracted with the advertising firm of Brown & Bigelow of St. Paul, Minnesota, to create sixteen paintings of dogs in various human poses. Nine of these paintings depicted dogs playing poker. On February 15, 2005, two of these much imitated paintings, A Bold Bluff and Waterloo went on the auction block expecting to fetch between $30,000 and $50,000 but surprisingly sold for $590,400.

The auction set an auction record for Coolidge, whose previous top sale was $74,000.