David Hendrik Jansen (born August 21, 1979) is an American mixed martial artist who formerly competed in Bellator's Lightweight division. A professional competitor since 2007, Jansen has also formerly competed for the WEC and M-1 Global.

Born and raised in Portland, Oregon, Jansen competed in high school wrestling. Jansen won a state championship in 1997, and also finished third twice. Jansen received a full scholarship to continue wrestling at the University of Oregon but only spent two years in college, having become "burned out" with wrestling. Jansen later joined Team Quest and transitioned into mixed martial arts.

Jansen held an undefeated amateur record of 6–0 before making his professional debut in September 2007, compiling a professional record of 12–0 with three appearances for the M-1 Global promotion before being signed by the WEC.

Jansen defeated Richard Crunkilton via unanimous decision in his WEC debut at WEC 43 on October 10, 2009.

Jansen suffered his first professional loss against Kamal Shalorus on January 10, 2010, at WEC 46.

Jansen next fought Ricardo Lamas on August 18, 2010, at WEC 50. He lost the fight via unanimous decision.

After his consecutive losses in the WEC, Jansen was released from the promotion.

Jansen made his Bellator debut at Bellator 39, defeating Scott McAfee by first-round submission.