Police say a lifeguard and other swimmers found the Lincoln man unresponsive Tuesday morning in the Cooper YMCA pool.
He was pulled from the water and lifesaving measures were performed until first responders arrived and took over.
The man was taken to a local hospital, where he was later pronounced dead. Authorities will not release his name until family has been notified.
The Lancaster County Attorney has ordered an autopsy in order to determine an exact cause of death.
The Cooper YMCA is located near Southwest High School.