The body was discovered by a member of the public in a private garden of a property in Victoria Quadrant, Weston-super-Mare, at about 08:50 GMT.
Police are treating the baby’s death as unexplained and are “extremely concerned” about the mother’s welfare.
Det Ch Insp Mike Buck said it was “very sad and distressing” and North Somerset Children’s Services appealed to the mother to come forward.
Calling for information, Det Ch Insp Buck said: “We’re extremely concerned for the welfare of the mother of this child and need to trace her as soon as possible.
“Our priority right now is to find the mother of this child and support her, so please do the right thing.”
Amanda Braund, from children’s services, urged the mother to come forward.
“We have specially trained professionals who can support you and make sure you get the medical attention you need,” she said.
“Please get in touch with the police by calling 999 or your nearest hospital.”
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