Reed Cox, 20, saved the woman with the help of two other boys last Thursday.
The incident occurred that evening, when Reed was on his way home after seeing friends and saw a car parked strangely on a bridge.
Reed's mum Tracey said: "There were two other boys there who were talking to a woman who was over the barrier of the bridge, and said they'd already called the police.
"Reed could see that she was lunging, and he managed to stop her from going over - I don't think he's ever run so fast in his life.
"She was obviously quite vulnerable and tried to push him off, but he held her until the police arrived.
"The boys just said 'well done you' to him as he left - he has saved her, and perhaps anyone who was under that bridge at the time.
"He is quite a shy, timid person, and when he came in and told me I was surprised. He was worried that she might take him with her, but thankfully that didn't happen - he just acted quickly."
Reed suffers from anxiety, brought on by complications following appendicitis some time ago which affected his school and work life.
Tracey said: "He has been trying to find something. He is such a nice, kind lad - if you need him he'll always do something for you.
"Young boys often don't tend to get a lot of positive press, but I am so proud of him."
A police spokesperson said: “We were called at about 11pm on Thursday, September 9, with reports of concern for a woman in Peterborough.
“The woman was found on a road bridge over the Frank Perkins Parkway (A1139) near Stanground and was taken to a place of safety.
“If anyone ever sees anyone in danger, such as in these circumstances, we would always urge them to dial 999 as soon as possible.”