David Atkinson, 46, jumped into the River Ouse in York and pulled the baby to safety after hearing her mother’s screams for help.
Mr Atkinson was helped by two other men on Thursday morning.
The 31-year-old mother was pushing her daughter in a pram near Lendal Bridge when she stopped to fasten the coat of her other child. As she did so, the pram rolled into the river.
Mr Atkinson, who was cycling home, stopped and leapt into the river, pulling the pram and child to safety. Police believe he almost certainly saved the baby’s life.
He said: “It felt like it was in slow motion – like something out of a film. I was so scared.
“I ran and tried to grasp the handles of the pram, but couldn’t and it went into the water. I thought there is nothing else to do but jump in after her. Yes, I was scared, but I had to try. Luckily she wasn’t under long. I then just gave the mum a hug – she was so upset. I think I said something daft like ‘just make sure she has a good life’.”
Police said the mother, who has not been named, was shaken and upset but the baby was not harmed.