The little boy was pronounced dead after he was discovered by the vicar of St Peter's Church, Bramley, Leeds, on Wednesday morning.
It's believed the tot was found as the vicar headed for a morning service. Police don't know if the baby was still alive when he was found in the Tesco bag-for-life at about 9.15am.
Detective Chief Inspector Nicola Bryar, of West Yorkshire Police, said her priority was finding the mother as she will need medical care and support.
Speaking outside St Peter's Church, Ms Bryar said: "I really want to appeal for the mother of that baby to come forward. She's our primary concern at the moment.
"We are very concerned for her welfare. We are sure that she'll be in need of medical attention.
"She will certainly need some support."
Officers said the vicar called police at 9.18am. The detective said she believed the baby, who was found unclothed in the bag with the placenta, was left during the hours of darkness.
Ms Bryar said the vicar's wife came home at 10pm the night before and did not notice the bag then.
She appealed for anyone who saw someone carrying a Tesco bag-for-life in the Hough Lane area between Tuesday night and Wednesday morning to come forward.
The baby was cold to the touch when found, she said.
Paramedics and doctors at Leeds General Infirmary tried to revive the youngster but without success.
But the pathologist had not been able to determine when the baby died.
She said the baby was white, male and full-term.
Ms Bryar said of the mother: "She's certainly gone through a traumatic ordeal. She's given birth and hasn't sought any medical attention. But it's the support as well."
Tributes including flowers, teddies and poems have been left outside St Peter's Church.
One message said: "Now you have your wings. Fly high into the arms of the Lord. God bless you little angel."
