Samantha Pendo was asleep in her mother's arms when police forced their way into their home and beat her and her parents as they searched for protesters, her parents said.
"She remained in coma throughout. She never improved one bit," said Dr. Sam Oula at the Aga Khan Hospital in the western city of Kisumu.
The baby and her parents were beaten when police were sweeping their neighbourhood for opposition protesters on Saturday, residents told Reuters journalists who investigated the incident.
Kisumu is a stronghold of opposition leader Raila Odinga, who is contesting results from last Tuesday's presidential election. An official tally said President Uhuru Kenyatta won re-election by 1.4 million votes.
Among the dead are an 8-year-old girl, hit by a stray bullet as she played on her balcony, and an 18-year-old student whose mother said was pulled from under the bed and beaten so badly he died the next day.
Police have promised to investigate all incidents but human rights groups say they rarely hold officers to account for extrajudicial killings.