Bill Russell was also a civil rights activist in addition to being the league's most crowned champion

Basketball is in mourning after Boston Celtics legend Bill Russell died aged 88 on Sunday.

Russell's family posted news of the passing on social media but did not reveal a cause of death. 

"Bill's wife, Jeannine, and his many friends and family thank you for keeping Bill in your prayers," said a statement.

"Perhaps you'll relive one or two of the golden moments he gave us, or recall his trademark laugh as he delighted in explaining the real story behind how those moments unfolded.

"And we hope each of us can find a new way to act or speak up with Bill's uncompromising, dignified and always constructive commitment to principle. That would be one last, and lasting, win for our beloved #6."

As their first-round pick out of the University of San Francisco, Louisiana native Russell helped build a dynasty in Boston where he played the full duration of his 13-year career from 1956 to 1969.

Winning 11 championships in 13 seasons, the last two of those wins came as a player-coach and as the first black head coach in major US sports.

To this end, NBA commissioner Adam Silver branded him "the greatest champion in all of team sports" but also noted that Russell, who marched for African-American civil rights with Martin Luther King, "stood for something much bigger than sports".

Silver credited Russell with stamping "the values of equality, respect and inclusion" into "the DNA of our league".

"At the height of his athletic career, Bill advocated vigorously for civil rights and social justice, a legacy he passed down to generations of NBA players who followed in his footsteps," Silver added.

"Through the taunts, threats and unthinkable adversity, Bill rose above it all and remained true to his belief that everyone deserves to be treated with dignity," he also noted.

Russell hung up his sneakers as an 11-time NBA champion, five-time MVP and 12-time All Star while his iconic number '6' jersey was retired in 1972 by the Celtics who have changed their Twitter profile picture to show it.

Also an Olympic gold medalist in 1956 with Team USA, Russell was voted the greatest player in the history of the NBA by basketball writers in 1980 as 2011 saw then-US President Barack Obama award him the Medal of Freedom and dub him a man who "stood up for the right and dignity of all men".

Russell passed away with his wife Jeannie by his side, and his family have said that arrangements for a memorial service will be announced soon.