Gregory J. Markopoulos (March 12, 1928 - November 12, 1992) was an American experimental filmmaker.

Born in Toledo, Ohio to Greek immigrant parents, Markopoulos began making 8 mm films at an early age. He attended USC Film School in the late 1940s, and went on to become a co-founder — with Jonas Mekas, Shirley Clarke, Stan Brakhage and others — of the New American Cinema movement. He was as well a contributor to Film Culture magazine, and an instructor at the Art Institute of Chicago.