Juan Carlos Aguilar is accused of fatally injuring his girlfriend's niece. Police said he often stayed at an apartment shared by his girlfriend and her 21-year-old sister, who is the baby's mother.
Police said they got a call about noon Saturday from Coastal Community Hospital regarding the suspicious death of the girl.
She was unconscious and not breathing when she was taken to the hospital by the two women.
"The broken ribs, bruises, renal hemorrhaging, those all made us come to the assumption that these were inflicted on the child and therefore that's why we made the arrest," said Cpl. Anthony Bertagna of the Santa Ana Police Department.
The mother told police she put the toddler to bed and found her unresponsive the next morning.
No charges have been filed against either of the women.
An autopsy to determine the exact cause of death is planned for Monday.
City News Service contributed to this report.