A one-month-old baby girl was discovered unconscious and with blunt impact trauma to her head in a Bronx apartment — and later died in what police have deemed a homicide, authorities said Friday.

Little Monreux Mercado wasn’t breathing when she was found at 4:15 p.m. Tuesday in a second-floor apartment building of NYCHA’s Butler Houses on Webster Avenue in Claremont, police and sources said.

The child was in her bassinet, and had signs of blunt force trauma, cops and sources added.

Medics rushed Monreux to Bronx Lebanon Hospital, but she could not be saved, police said.

The city’s Medical Examiner ruled that the baby died of a “blunt impact of [the] head with skull fracture.”

Monreux’s 35-year-old mother made the 911 call about her unresponsive baby, police said.

The mom told cops she put the baby down while she got ready for an appointment. She said her friend, who was also in the apartment, then noticed the baby wasn’t breathing, according to authorities.

No arrests have been made.

Neighbors at the public housing complex said Monreux’s family moved in about a month ago.

“It’s very devastating to find something like that happen. This is a tight knit community and it’s very tragic to see,” Butler Houses tenant association president Cornell Nolton said.

Nolton added that he heard “somebody saw the baby in the hallway and someone tried to give it mouth to mouth.”

Chanel Caraway, a spokeswoman for the city’s Administration for Children’s Services, said the agency is investigating the case with the NYPD and has “taken action to secure the safety of the other children in this home.”

Additional reporting by Natalie Musumeci