The 38-year-old captained his country for a decade and scored 325 goals in 737 appearances for 10 different clubs during a 23-year playing career.
“Today, after 23 wonderful years, I am formally announcing my retirement as a professional footballer,” Keane said in a statement.
“From Crumlin United around the world to the likes of Milan, London and Los Angeles, I could never have imagined the path my football life would take. It has exceeded all the hopes I had as a football-mad boy growing up in Dublin.”
Keane played for the likes of Wolves, Tottenham, Inter Milan, Leeds, Liverpool and LA Galaxy after arriving in England aged 15. He is also the Republic’s all-time leading scorer, netting 68 goals in 146 appearances before retiring from international football in 2016. The Republic’s new manager, Mick McCarthy, appointed Keane as his assistant last week alongside Terry Connor following the recent departure of Martin O’Neill.
“Playing for and captaining my country has been the highlight of my career,” he said.
“I enjoyed every moment of the most incredible journey with the Ireland team over an 18-year period and as I have said many times before, words cannot express how proud I am to be Irish.”
Keane arrived in England with Wolves in 1997 and moved on to Coventry for £6m after two seasons in the first team.
Further moves followed, to Inter Milan (£13m) and Leeds (£12m) before he spent six years at Tottenham.
Liverpool paid over £20m to sign Keane from Tottenham in 2008, but he returned to White Hart Lane six months later.
Following loan spells at Celtic and West Ham he moved his family to Los Angeles, where he spent “five hugely successful and rewarding years, both on and off the field, with LA Galaxy”.
Keane added: “To all those who believed in me, supported me and picked me up when I needed a hand, you can never know how much that has all meant. It was an honour to represent you.”