A major search was launched by police after a member of the public reported finding the baby girl in Watson Road Park in Blackpool at about 7am on Sunday.
Police said an upturned pram was discovered with a baby lying in the mud, and a handbag nearby but no sign of the child’s mother.
Emergency services searched for two hours to find the baby’s mother, before finding her with her other six-year-old daughter in Blackpool.
Police said the baby was safe and well, and officers were working to find out how the baby came to be in the park.
They said a woman was “helping with inquiries” on Sunday afternoon.