West Yorkshire Police said officers received a report of “concern for the safety of a young child” in Leeds at 11.06am yesterday.
When emergency services attended a property on Austhorpe Court, the three-year-old girl’s body was found inside.
Detective Chief Inspector Stacey Atkinson, of West Yorkshire Police’s Homicide and Major Enquiry Team, said: “A woman at the address was taken to hospital for urgent medical attention and remains there today in a stable condition.
“She has been arrested on suspicion of murder.”
She added: “We are treating this as an isolated incident and are not seeking anyone else in connection with it.
“I understand that such a tragic incident as this will cause shockwaves in the local community and that there will be speculation about what has happened.
“I would ask people to please let us investigate and if you do have any information that you think could assist our enquiries to get in contact with the investigation team.”
Anyone with information is asked to contact the Homicide and Major Enquiry Team via the live chat facility on the West Yorkshire Police website or by calling 101, quoting crime reference 13250433711.