Emergency crews responded to the park near 90th Street and West Drive just after 1 p.m.
An Eyewitness News crew saw rescuers pull the person from the reservoir, and a police helicopter was also flying overhead.
He was taken to St Luke's Hospital, where he was pronounced dead.
Several people posted video and photos to social media showing the activity:
It is unclear how the person ended up in the water, but at this point, it is being investigated as a suicide after several witnesses reported the man jumped.