A man is behind bars after he allegedly beat his teenage cousin with a belt at least 50 times and left him to drown in an Upper West Side bathtub.

Mitcaux Booker, 32, told detectives that he repeatedly lashed Jallen Mcconnie, 15, on his arms, legs, body and face over the course of an hour on June 28, before leading him to the tub of a W. 94th St. apartment, court records show.

Booker told cops he saw Mcconnie “buckle” and slump down in the tub with the water running. He returned two hours later to find the teen unconscious in the tub, according to a criminal complaint filed in Manhattan.

When the FDNY arrived, the severely bruised unconscious 15-year-old was lying on the bed wearing dry clothes, the complaint said.

Mcconnie, who was visiting family in New York from Georgia, according to police sources, was pronounced dead at Mount Sinai Hospital that night, records show.

Booker was charged with assault and endangering the welfare of a child on July 1, Manhattan prosecutors said.

He was indicted by a grand jury on an unsealed charge on July 2 and is being held in jail ahead of an arraignment in Manhattan Supreme Court next month, according to the district attorney’s office.