He became a consistent member of the Ireland team until the 2004 Six Nations when he broke his arm in the match against Italy. This was followed by a serious leg injury during a training exercise in August of the same year.
Despite being a potentially career-ending injury, Gleeson applied himself to making a full recovery and marked his return to the game with an appearance for his club, St. Mary's College, late in the 2004/05 season. He went on to reclaim his place in the Leinster senior team and make a further four appearances for his country, bringing his total number of Irish caps to 27.
Commenting on his decision to retire, Gleeson said: "I have thoroughly enjoyed my time with Leinster Rugby and am honoured to have played for both my province and country. My decision to retire is largely inspired by my desire to explore new avenues and opportunities and I feel now is the time to do that. I would like to take this opportunity to thank my coaches, all the support staff at Leinster, my friends and family for their support over the last seven years."
Leinster coach Michael Cheika said: "Keith Gleeson has been an incredible ambassador for Leinster and Irish rugby. He is the embodiment of professionalism and his work ethic, skill set and the pride with which he plays and prepares for games are second to none. I know how tough he is. I've played against him and coached him and every time he plays the game he tries to play his best match. That takes courage.
"We are disappointed that he is leaving, but he goes with the best wishes of everybody connected with Leinster. He has played an important role in helping the next batch of back rows to develop within the province and I am sure that he will be determined to go out on a real high in the remaining few matches of the season.
"I know how hard a choice it has been for him to make because he has great desire to wear the blue shirt and the green shirt which both mean so much to him. But we respect his decision and can only thank him for his contribution to the province and to his help to me during my three years with him at Leinster.
"I wish Keith and his wife Fiona all the best for life after rugby - I know that it will be success just as it has been in rugby," he added.
Leinster Chief Executive Mick Dawson said: "Keith has been an integral part of the Leinster squad since first arriving and we are absolutely sad to see such a talent depart.
"His contribution to the game within the province has been immense and if any youngster is looking to take up the game, then they should look at the professional and dignified manner in which Keith has handled himself over his career.
"Keith showed immense courage in recovering from what was a potentially career-threatening leg injury and his hunger and appetite to compete is an example to any youngster taking up the game.
"We are sad to see him go because his form has been of a particularly high standard yet again this year, but we wish him well in his retirement."
Born in Dublin, Gleeson emigrated to Australia at the age of seven only to return in 2001 as a result of an invitation to play with Leinster. While in Australia he played with the New South Wales Waratahs, Australian Barbarians, Australia Under-19s (1995) and Australia Under-21s in 1996 and again in 1997 when he was captain.
He will return to Australia at the end of the year with his wife Fiona to work in the family Financial Planning business.
Position: Openside flanker
Club: St. Mary's College RFC
School: St. Aloysius College, Sydney
Date of Birth: June 17, 1976
Height: 6' 1"
Official Leinster Caps: 115
Points Scored: 60 (12 tries)
Magners/Celtic League/Cup Caps: 70 (7 tries)
Heineken Cup Caps: 43 (5 tries)
Senior Debut: Celtic League, 39-11 win v Glasgow at Donnybrook, August 17, 2001
Heineken Cup Debut: 40-10 win v Toulouse at Donnybrook, September 28, 2001
Ireland Caps: 27 (February 2002 -)
Ireland Points: 25 (5 tries)
Ireland 'A' Caps: 6
Other Honours: Australia Barbarians, Australia Under-19s (1995), Australia Under-21s (1996 and 1997 (captain))