Former Maroons and Kangaroos centre , effective immediately, due to a chronic knee injury.
The dual premiership-winning Cowboys veteran has battled knee injuries for much of the later years of his career, but it has now deteriorated to the point where he can no longer take the field.
In 2019, he ruptured his spleen in the round-11 clash with the Raiders and spent four days in intensive care in a Canberra hospital after emergency surgery - which included three blood transfusions - saved his life.
He had returned to his team hotel and gone to bed after the game unaware of the dire situation he was in. It was his roommate, his long-term wingman Kyle Feldt, who noticed his precarious condition and the ambulance was soon called.
The 30-year-old was an integral member of the Cowboys’ historic maiden premiership in 2015, scoring 13 tries in 26 games.
He crossed for two tries in North Queensland’s resounding 38-4 World Club Challenge victory over Leeds at Headingley in February, 2016.
The Cowboys draw level with O'Neill try
O’Neill cemented his standing as one of the NRL’s premier centres in 2016 with State of Origin selection for Queensland and two Tests for Australia in the victorious Four Nations campaign.
In 2017 he again starred during the Cowboys’ fairytale run to the grand final and started at centre in the decider against the Melbourne Storm.
Injuries limited O’Neill to just 30 games over the past three seasons, although he did manage an impressive 15 tries in those matches.
Match Highlights: Broncos v Cowboys
O’Neill’s 122 appearances rank 22nd in club history, while his 44 tries are the 13th most in Cowboys colours.
Prior to joining the Cowboys, O’Neill appeared in 67 games for Melbourne, including the 2012 grand final victory over the Bulldogs.
"I am disappointed my career has come to an end. Starting out in 2010 I had a small goal in my head which was to hopefully play until I was 30 and I’ve done that. I am very grateful for the career I have had and the success and friendships I have gained along the way," O’Neill said.
"I have had some amazing experiences which were only made possible through a career in the NRL and I’m very thankful for everything the game has given me over the years.
"I want to thank everyone who has played a part, big or small in my career, including the two clubs I played my footy with – the North Queensland Cowboys and the Melbourne Storm.
"I would like to express a special thank you to the Cowboys for the opportunity in 2014 to further my career and the support they have given me and my family over the last seven years.
"Lastly, I would like to say a huge thank you to my family, especially my wife for all her support and my parents for everything they did to give me the best opportunities growing up.
"I am looking forward to my next chapter in life and taking on new roles in whatever lies ahead."
Match Highlights: Bulldogs v Storm
Cowboys head of football Micheal Luck said O’Neill was an immense contributor to the club on and off the field.
"We are devastated Justin has had to retire early due to injury," Luck said.
"Justin has given terrific service in his seven seasons at our club. He was a class player and a massive contributor to the 2015 premiership and the golden run the club had during that period.
"The way he has handled the past couple of seasons as he has battled knee injuries has been brave and has set a terrific example for our younger players with his commitment to preparation and recovery."