Douglas Seneca Gjertsen (born July 31, 1967) is an American former competition swimmer, Olympic champion, and former world record-holder.

Gjertsen was the third member of the record-setting U.S. team in the men's 4×200-meter freestyle relay at the 1988 Summer Olympics in Seoul, South Korea.

He also received a bronze medal for swimming for the third-place U.S. team in the preliminary heats of the men's 4×100-meter freestyle relay.

[1] Four years later at the 1992 Summer Olympics in Barcelona, Spain, he was the fourth member of the third-place U.S. team in the men's 4×200-meter freestyle relay; the American team of Joe Hudepohl, Mel Stewart, Jon Olsen and Gjertsen won the bronze medal with a time of 7:16.23.

Gjertsen has been instrumental in the orchestration of the Atlanta Swim Across America event which he helped kickstart in 2015.