PC Ed Barker, of Greater Manchester Police (GMP), stopped the man falling 40ft (12m) from Rochdale Town Hall.
Seeing the man in distress, PC Barker ran up the building's fire escape steps before catching him just as he tried to throw himself to the ground.
PC Barker has been awarded the force's John Egerton Trophy.
Chief Constable Ian Hopkins presented the trophy, which is awarded annually to a GMP officer who has performed an exceptional act of valour.
The award is named after PC John Egerton, who was killed in the line of duty on 11 March 1982, aged 20.
PC Barker is also due to meet Prime Minister David Cameron during the Police Bravery Awards on 14 July.
Mr Hopkins said his officer showed "incredible bravery" during the incident, which happened in January 2015.
"There was a genuine risk of him being carried over the edge of the snow-covered fire escape, which could have had tragic consequences for both men," he said.
"His cool head and quick thinking saved this young man from certain death and I am delighted that he is now safe and well."
PC Barker said the 21-year-old man's friends had tried to help him but were "losing their strength" when he arrived.
"I managed to get hold of him around the neck and shoulders because the rest of his body was hanging off the edge of the fire escape," he said.
"I was so relieved when I saw the blue lights arriving to help take him to hospital.
"It's nice to get some recognition and appreciation, but I was helped by his friends and my colleagues."
A police officer who saved the life of a suicidal man by grabbing him by the neck as he tried to jump from a snow-covered roof has won a bravery award.