A Brooklyn mother charged in the beating death of her 4-year-old son allegedly admitted she killed the boy on Thursday, a police source said.

Zarah Coombs, 26, apparently told investigators she beat young Zamair Coombs with a stick before he was found unconscious in a storage tub filled with water inside the family’s basement apartment on Riverdale Avenue in Brownsville about 9 p.m. Wednesday, police said.

Coombs’ 1-year old and 1-month-old children were home at the time but have since been transferred into the custody of The Administration for Children’s Services, the source said.

Zamair was taken to Brookdale University Hospital and Medical Center, where he was pronounced dead.

Zamair’s death was ruled a homicide on Thursday from “multiple blunt impact injuries,” according to the medical examiner’s office.

Coombs was charged with murder, manslaughter, acting in a manner injurious to a child and criminal possession of a weapon, police said.

Cops questioned the boy’s 26-year-old stepfather, but were no longer interviewing him by Thursday afternoon.

Coombs has one prior arrest for shoplifting in 2015, a police source said.