Alexander Cowan, 62, said he felt a sense of injustice when he saw the six-strong gang assault the man and take his glasses in an Edinburgh underpass.
And even though he had been diagnosed with diabetes only a month before, he instinctively stormed into the melee, was punched to the ground, but still managed to get up and "nearly put a couple of them in hospital".
Despite standing at only 5ft 6in, Mr Cowan was able to hold one of the gang until police arrived.
The Fountainbridge man was today due to receive the prestigious St Andrew's Award from First Minister Alex Salmond at Edinburgh Castle for his bravery.
The drama happened as Mr Cowan walked home with his wife Anne, 49, after an evening at the bingo at Fountain Park in February last year.
As they approached the Telfer Subway, Mr Cowan's patience snapped as he watched a man in his 20s get assaulted "for no reason at all".
"I'd just had enough," he said. "I was sick of guys like that running around the streets as if they ruled the roost.
"I ran straight over, and was knocked to the ground at first and then my wife faced them down as I got up.
"I'm not the biggest guy in the world, but I've done a bit of boxing in the past, so I just fought back.
"They weren't expecting it and I nearly put a couple of them in hospital. Five then ran off and I managed to keep one there until the police came.
"They're not the brightest guys in the world and as the police came down one of the five walked right past so I was able to point him out."
It later emerged that the youths, who were part of a larger group of 20, had embarked on a campaign of violence in the area that night, attacking passers-by and vandalising property.
Mr Cowan had expected that to be the end of the matter, and was even jokingly ticked off by police who warned him some of the group had weapons.
"I didn't know that at the time and you don't think," he added. "I'd only been diagnosed with diabetes a few weeks before and my doctor told me not to exert myself.
"But I wanted to, they'd set upon this guy for no good reason. I don't think enough people get involved when things like this take place. You see it all the time, they just walk straight past, but I'd do it again."
Today's ceremony is part of the [email protected] awards, which recognises 50 people across the country for similar feats.
Of those, a handful like Mr Cowan are set aside for the St Andrew's accolade.
He added: "It was a real surprise, it was months later when I got the call because the police had nominated me.
"You don't expect it, but it's a nice thing for them to do."