PALM BEACH GARDENS, Fla. — Ron Palillo, best known as mouthy classroom goofball Arnold Horshack on the 1970s TV series “Welcome Back, Kotter,” died at his Palm Beach Gardens home early Tuesday morning, according to Stacy Sacco, Palillo’s friend.

Sacco said Palillo passed away suddenly at 4:30 a.m.

He was 63.

“He just couldn’t have been more fun and intelligent or talented. He was an amazing human being,” said Sacco, who met Palillo 20 years ago when he was a guest on Ellen DeGeneres’ sitcom in Los Angeles, on which Sacco was a script supervisor.

“I had been a fan. We all knew who he was. He was a shining star,” Sacco said.

Palillo had been teaching acting classes at G-Star School of the Arts, a charter school in Palm Springs, for about three years.

Sacco said that no cause of death has been determined, but that Palillo had not been ill or in the hospital.

Born April 2, 1949, in Cheshire, Conn., Palillo is survived by his partner of 41 years, Joseph Gramm, a retired actor.

Palillo attended the University of Connecticut and had lectured in colleges and high schools all over the country before coming to Palm Beach County.

Palillo played Horshack on the popular sitcom, which also starred Gabe Kaplan and young John Travolta. It originally aired on ABC and ran from 1975 to 1979.

The show focused on Kaplan’s character, Gabe Kotter, who returned to his high school in Brooklyn — the fictional James Buchanan High — to teach a remedial class to which he once belonged.

Palillo’s character belonged to an unruly group of students known as the Sweathogs, who always got in trouble. Palillo played the sheepish Arnold Dingfelder Horshack, who would often yell “Ooh! Ooh, ooh!” when he thought he knew the answer to a question.

Palillo said his fellow “Kotter” actors paid careful attention to their characters. “The writers asked us to write our characters’ autobiographies as our characters,” Palillo told the Post in 2009. “I wrote how Arnold’s mother had been married so many times and how he just wanted to be liked and how no one was going to read this autobiography he was writing. The writers used all of it.”