The ferry was moving from the mainland to the island at the time of the incident.
Likoni police boss Willy Simba said the deceased is yet to be identified as he did not have a ID card on him.
Simba said Kenya Ferry Services divers' attempts to rescue the man after he jumped into the waters failed.
"They want after him immediately, pulled him out of the water and administered first aid but it was already too late," Simba said.
He said the body was taken to Coast general hospital mortuary.
Cases of people attempting suicide have been rampant at the channel in the recent months.
Simba associated the cases to marital problems and challenges associated with high cost of living. He urged people to seek assistance from counselors instead of resorting to suicide.
Another man was recently arrested for attempted suicide in the same manner. Kyalo Mwanzui, 23, jumped from the ferry at the Likoni channel but was rescued by the Kenya ferry services rescue team.
In yet another incident, a Kiambu man said that God had sent him to jump off the ferry. Muigai was arrested on May, 31, 2016 and arraigned in court for attempting to take his life by jumping into the Indian ocean from a moving ferry.