A Jewish New Year celebration turned tragic when a teenager woke in a Brooklyn home to find his 1-year-old brother drowned in a toilet, police sources said Tuesday.

The body of little Mendi Altman was found by his 13-year-old brother at an aunt’s 11th Ave. home in Borough Park late Monday night.

Police sources said the accident happened after everyone had gone to sleep for the night.

“(All the kids) were checked on,” said one source. “Later, one of the siblings goes to use the bathroom … and finds the child unresponsive in the toilet bowl.”

A Hatzollah ambulance took the baby to Maimonides Medical Center, where he was pronounced dead.

The body showed no signs of trauma and the boy had not been ill, sources said.

The boy, who lives with his parents and six siblings in largely Hassidic upstate Monsey, was visiting his aunt to ring in the year 5773 on the Jewish calendar.