A jury at Kingston Crown Court took nearly three and a half hours to return its verdict in favour of the the 16-stone (102kg), 6ft (1.8m) forward who had denied unlawfully and maliciously inflicting grievous bodily harm on Stefan Marty during a match with London Irish in January.
Outside the court, Mr Rees - who plays for Nottingham and has been capped 23 times for England - broke down and sobbed.
During the four-day trial, David Jeremy, for the prosecution, claimed Mr Rees, 32, landed a punch from behind on the side of Mr Marty's face following a line- out.
Mr Marty, 31, a physical education instructor at Wimbledon College, said he had no warning of the blow. He suffered a compound fracture of the jaw, lost two teeth and 1 1/2 stone (9.5kg) in weight, and has not played since.
Mr Rees, of Kegworth, Leicestershire, claimed Mr Marty persistently fouled him during the match and to gain the referee's attention he decided to 'create a situation' and become entangled with him. He pushed Mr Marty with his left hand and then brought his right arm round in a 'sweeping motion' to 'barge' his opponent to the ground.
Mr Rees said he was never involved in foul play, never intended to hurt Mr Marty and was 'distraught' at his injury.
A statement issued on Mr Rees's behalf said the player was 'relieved' that the jury accepted Mr Marty's jaw was broken accidentally and that he 'never condoned and never will condone' violence on or off the pitch.
'It would be unfortunate if the spirit of the game, the important element of sportsmanship and in addition the authority of the referee were to be undermined by resorting to proceedings of this nature in the future when they are properly the province of the governing body of the sport,' he said.