An 8-month-old Harlem girl was in grave condition Tuesday with no brain function after her mother left her unattended in a filling bathtub, police sources said.

The infant, whose name wasn’t released, was found face down in the water around 4:30 p.m. Monday in an apartment at Frederick Douglass Boulevard and West 124th Street, cops said.

The mother told cops she had run the water after putting the infant upright into her bath seat — but then left the room because the water wasn’t filling up. She told police that when she returned, the baby was face down in the water because her seat had flipped over, the sources said.

The father tried CPR but the baby couldn’t be revived, cops said.

She had no brain function and was considered “likely to die,” according to police sources.

An ambulance rushed her to Harlem Hospital, and she was transferred to Columbia Presbyterian Hospital.

Police were investigating possible child neglect.