A former actor suspected of stabbing one of his instructors in tai chi class in Massachusetts has been arrested, police said.

Henry Tomaszewski, 62, of Malden was busted late Wednesday while sitting on a porch in Cambridge, authorities said. His arrest comes one week after a 60-year-old martial-arts instructor told police he was stabbed in the back of the neck by the onetime actor with a jackknife, police said.

Cambridge police spokesman Jeremy Warnick said the attack happened after an altercation at the city’s YWCA on Temple Street on Aug. 21.

Police said it’s unclear what motivated the attack, but the victim told investigators that Tomaszewski came up behind him as they did non-contact warm-ups and stabbed him in the back, according to a criminal complaint obtained by The Post.

The instructor told investigators that he only knew Tomaszewski – who appeared in 1979’s “Billy in the Lowlands” and three other 1980s flicks, according to his IMDb profile – by his first name and said he was the class “charity case,” included only to be kept off the streets, the complaint continued.

Tomaszewksi had been circling the class in an unsettling manner just prior to the unprovoked stabbing, the lead tai chi instructor told police. After the stabbing, he appeared to be confused and put the knife down before picking it back up and running off, according to the complaint.

The victim was taken to a hospital and treated for minor injuries, Warnick said.

Tomaszewski was arraigned Thursday in Cambridge District Court on a charge of assault and battery with a dangerous weapon on a person over the age of 60, a spokeswoman for the Middlesex County District Attorney’s Office told The Post.

A judge set his bail at $500 cash and ordered him to stay away from the victim and the YMCA, as well as to not possess any weapons. Tomaszewski was also ordered to wear a GPS monitoring bracelet. He was released and is expected to return to court Sept. 13, spokeswoman Liz Vlock said. The suspect could not be reached for comment.