Niemann alleges Carlsen paid another player €300 to shout “Cheater Hans” from a public balcony during one his matches. Carlsen has not publicly responded to the accusation.
The claim is made in a federal lawsuit that Carlsen, chess.com and others destroyed his career by falsely accusing him of cheating.
Niemann is seeking $100m in damages in the lawsuit filed in US District Court in St Louis, elevating a controversy that has roiled the chess world since September, when Carlsen suggested Niemann cheated during his upset win over Carlsen at the Sinquefield Cup tournament in St Louis.
The lawsuit names Carlsen; his company Play Magnus Group; online chess site Chess.com; Hikaru Nakamura, an American grandmaster who is an influential streaming partner on Chess.com; and Chess.com executive Danny Rensch.
It alleges the defendants worked together to defame and libel the 19-year-old Niemann after his Sinquefield Cup win.
“Despite the falsity of the Defendants’ accusations, Defendants' malicious defamation and unlawful collusion has, by design, destroyed Niemann’s remarkable career in its prime and ruined his life,” according to the lawsuit.
Niemann says tournaments have banned him, matches have been canceled and reputable chess schools won’t hire him over the false accusations.
The lawsuit contends Carlsen is seeking to preserve his status as the “King of Chess” and to complete a deal with Chess.com to acquire Play Magnus for nearly $83m.
Carlsen — a 31-year-old world champion who is generally considered one of the greatest chess players ever — has not commented on his social media posts about the the lawsuit. His manager, Henrik Carlsen, did not immediately respond Friday to a request for comment.
Attorneys Nima Mohebbi and Jamie Wine said in a statement posted on Chess.com's website that Niemann's lawsuit will hurt the game of chess, its players and its fans.
“There is no merit to Hans’ allegations, and Chess.com looks forward to setting the record straight on behalf of its team and all honest chess players,” according to the statement.
After Niemann's surprise win over Carlsen in the Sinquefield Cup, Carlsen abruptly withdrew from the tournament and a week later stopped playing after making only one move against Niemann at the Julius Baer Generation Cup.
Niemann has previously admitted to cheating when playing online chess when he was 12 and 16, but has denied cheating since then and said he never cheated during in-person matches.
On Sept. 26, Carlsen said in a statement on Twitter that he believed Niemann "has cheated more — and more recently — than he has publicly admitted.”
But Carlsen offered no evidence of Niemann cheating. The lawsuit contends several independent sources and experts found no evidence that Niemann cheated in any of his games against Carlsen, including at the Sinquefield Cup.
In a statement published in September, the president of the International Chess Federation, Arkady Dvorkovich, said the governing body shared Carlsen’s “deep concerns about the damage that cheating brings to chess” and is prepared to investigate incidents “when the adequate initial proof is provided.”
Additional reporting by AP