Sonny Bill Williams [Source: Sporting News]

Another sports star has today announced his retirement.

Just a day after Cameron Smith revealed he is officially done playing rugby league, Sonny Bill Williams has posted a heartfelt farewell on social media.

SBW will continue his professional boxing career but has decided to hang up his boots after a glittering career in the rugby union and rugby league arenas.

The 35-year-old, finished the NRL season with the Sydney Roosters last year.

He burst onto the scene as a teenager in 2004, winning an NRL title with Canterbury in his debut season.

SBW represented the All Blacks at three Rugby World Cups.

Williams was only the second player selected for the All Blacks after having represented the Kiwis at Test level rugby league and was one of just 14 players in rugby union history to have won successive World Cups.

He was also a member of the New Zealand Rugby 7s team at the 2016 Olympics in Rio.

Williams played only five matches with the Roosters last year, finishing his NRL career with 123 matches across eight seasons in three stints.

He played 12 matches for the New Zealand rugby league side and 58 appearances for the All Blacks.