St Helens half-back Lance Hohaia says he is retiring from rugby league because of "recurrent post-match concussion-type symptoms".
Hohaia, 32, lasted just two minutes of last year's Grand Final after being punched twice by Wigan prop Ben Flower.
Flower was given a six-month ban for the assault.
In a statement, Hohaia said the symptoms have occurred "since returning to play after the off-season and this is the reason for my retirement".
He added: "I am currently seeking expert medical advice to understand any potential effects to my long-term health.
"However, in the meantime I would like to say that I am extremely grateful for the career I have had and the opportunity to play for my country and such prestigious clubs as St Helens and the NZ Warriors.
"I would also like to sincerely thank all those who have supported me throughout my career and during this difficult time."
St Helens won last season's Grand Final at Old Trafford, although Hohaia was unable to continue in the match after the incident involving Flower.
The former New Zealand international accepted a one-game ban for striking Flower with his forearm, which led to the Wigan prop's retaliation.
Hohaia has featured eight times this season for Keiron Cunningham's side, scoring two tries.
The 28-cap international joined St Helens before the 2012 season on a four-year contract.
In a statement, the club said: "Lance has been a valued and integral member of the squad for the last three and a half years and we thank him for his service.
"We wish him and his family well in their life after rugby league with St Helens.
"St Helens will make no further comment on this matter."