The PFA's longest serving president made the announcement to bring to an end an outstanding playing career that spanned 18 years.

Capped 26 times for the Socceroos, 35-year-old Colosimo has won the A-League premiership and championship and was awarded the Joe Marston Medal on two occasions.

In accordance with the rules of the PFA, his retirement will see the former Manchester City player step down as president with immediate effect.

Having joined the PFA executive committee in 2002 Colosimo became president two years later, taking over from Socceroos legend Alex Tobin.

During his tenure as president, Colosimo played an integral role in a number of the PFA's key achievements, including:

:: The A-League's first Collective Bargaining Agreement (CBA).

:: The Matildas first ever CBA.

:: The establishment of the Alex Tobin OAM Medal – the players' highest honor.

:: The introduction of Minimum Medical Standards for the A-League.

:: The PFA taking control of player development and wellbeing programs.

:: Three Socceroos CBAs.

"The time is now right for me to move on with the next stage of my life," said Colosimo.

"During my career I was privileged to have played the game at the very highest level and to fulfil a lifelong ambition of representing my country.

"I have also been fortunate enough to have played with many of the greats of Australian football, including the likes of Vince Grella, Harry Kewell, Mark Schwarzer, Alex Tobin, Mark Bresciano and Tim Cahill. I have also been able to witness first hand the impact these players and their peers have had on the game.

"I emerged into professional football at a time when the sport was widely regarded as a joke in this country. This is now far from the case, with the Socceroos and Matildas consistently showing they can compete with the world's best and the A-League transforming domestic football in this country.

"Whilst this is hugely encouraging there remains much work to be done to ensure football can fulfil its enormous potential in Australia. This is a challenge that confronts all of the game's stakeholders and one that the players will continue to embrace."

Socceroos great and Colosimo's former teammate Grella said Colosimo would leave a lasting legacy in Australian football.

"I have known Simon since we played together as 12-year-olds," Grella said.

"He has had a hugely successful career and it could have been even more so had circumstances on occasions not gone against him.

"I have seen on countless occasions just how much passion he had for his fellow players and how hard he worked to ensure their welfare. The players have been very fortunate to have had someone like Simon representing them."

Current Socceroos midfielder Mark Bresciano also paid tribute to Colosimo.

"We were together at the AIS, Carlton and the national team," Bresciano said.

"It has been a privilege to play alongside a player and person like Simon.

"He has brought so much to the game. He cares deeply about the players and has always worked tirelessly on their behalf and to build the game in Australia."

Despite stepping down as president, the invaluable knowledge Colosimo has gained throughout his career will not be lost to the PFA, with chief executive Adam Vivian announcing that the inaugural Melbourne Heart captain will take on a staff role.

"The best example of leadership is leadership by example and Simon's commitment and passion to support the players, both in Australia and internationally, are nothing short of inspiring," said Vivian.

"Whilst the PFA is sorry to lose the services of Simon as president, I am delighted to announce that Simon has accepted a role as PFA player relations executive, where his unique understanding of the pressures that professional footballers encounter in their careers can still be utilised.