Melissa Goudie was cycling along the towpath behind the Crowne Plaza Hotel near Reading's Caversham Bridge on Saturday with seven-year-old son Alex and her friend, Sandra Bungay, when she spotted the child floating face down in the water.
Describing how the terrifying incident unfolded at around 12.30pm, she said: "I suddenly saw this jacket disappear beneath the water. I looked around and saw a man nearby with another child, and I shouted at him, but it all happened so quickly.
"I immediately jumped off my bike and ran to the edge of the water, and saw the little boy."
Acting on instinct, Melissa, 35, from Tilehurst, jumped in to grab the boy, who looked to be about two or three and had floated out about nine feet from from the bank.
She said: "I was obviously fully clothed, I had a fleece, a jacket and trainers on, but I swam to him and pulled him back to the edge. He was wearing a padded jacket, which I think helped him from sinking, but he was just going under water when I reached him. I was in the right place at the right time really. I didn't think, I just completely acted on impulse."
The Royal Berkshire Hospital medical secretary was pulled from the water by Sandra and some joggers who had stopped to help, but she said that the man believed to be the little boy's father said a quick thank you before walking off.
Melissa said: "I called my husband Glenn to bring me some dry clothes, but the staff at Crowne Plaza very kindly let me use their facilities to shower and get cleaned up because I was wet and quite cold as the adrenaline wore off!
"It was very much a double act because I knew Sandra was waiting on the side and would be able to help me back out. There was no question of standing there and letting him drown."
Melissa, who is also mum to 10-year-old Hannah, has been described by Glenn as a "real hero." He added: "Melissa is rather reluctant to be called a hero but she truly is, and it shows that there is still kindness in today's broken society."