The officer has been with the force for less than two years and was armed only with his baton when he confronted the three men during the rampage.
His condition is now stable.
Chief Constable Paul Crowther said:
“Having visited the officer in hospital shortly after he was admitted for treatment, I was able to hear his account of what happened last night.
“It became clear that he showed enormous courage in the face of danger, as did many others who were at the scene and rushed to help.
“Although he is seriously unwell, he was able to recount how he faced the attackers armed only with his baton, outside London Bridge station.
“For an officer who only joined us less than two years ago, the bravery he showed was outstanding and makes me extremely proud.
“All of us at BTP wish him a swift recovery, and I know he will be touched by the hundreds of messages of support from across the UK and the world.
“Our thoughts are with all of those who died or were injured, and their loved ones as they try to come to terms with what happened.”