Steve Canny was watching television with his girlfriend when he heard two crooks smash the windows of their £26,000 Mercedes.
Dressed in just his boxers, the ex-number eight ran outside and shouted at Leon de Silva: “You picked the wrong car!”
The powerhouse then wrestled de Silva, 19, to the ground and pinned him down as the teenager vainly tried to struggle free.
The 33-year-old told him: “Stay there! You ain’t going nowhere, mate! You broke into my car. You must be stupid.”
De Silva, who suffered two black eyes in the clash, was arrested and later admitted a string of offences. He is now serving 18 months in a young offenders institution.
The banker’s girlfriend, who asked not to be named, told the Express: “My boyfriend used to play rugby so this guy didn’t have much of a chance.”
Mr Canny, who played for Campion RFC in Romford, Essex, is now executive director of Volcap Trading.
Police praised his courage in tackling and detaining the thief. A source said: “He’s a real hero. De Silva certainly picked on the wrong man.” Mr Canny declined to comment.