Police said he was discovered in the pool at Franklin Park in the 4300-block of West 15th Street just before 1 a.m. Monday.
Paramedics performed CPR on him, but were unable to revive him. Investigators said the man may have drowned. His identity has not been released.
Police say the victim's girlfriend told them he had been smoking an unknown substance and drinking heavily before jumping in the pool.
Chicago's pools just opened up on Friday.