Attacks on Jews in New York more than trebled last year, according to figures released by the Anti-Defamation League.
There were 22 assaults on Jewish New Yorkers in 2013, compared to just six in the previous 12 months.
The ADL said it could not explain the significant rise, but it is thought the "knockout game" trend was partly responsible.
As part of the knockout stunt, an unsuspecting victim is punched by an attacker who films the assault and then posts footage on social media sites.
The craze is similar to the "happy slapping" trend in Britain a number of years ago.
Although the number of physical assaults in New York rose, general antisemitic incidents including graffiti and harassment, fell across the city, and the United States, last year.
Evan Bernstein, the ADL's New York regional director, said: “What we have seen in the past is that when things happen in Israel, for example, it could stir things up here. But that wasn’t the case last year.
"The rise of assaults in certain areas of New York City - in particular Brooklyn - is both disturbing and a sobering reminder that antisemitism is not just history but remains a current event.”
One incident involved a 12-year-old girl being punched in the head, and in another a Jewish pensioner was punched in the face.