A 33-year-old woman died after leaping from an Upper East Side apartment building Sunday afternoon, law-enforcement sources said.

The unidentified woman was pronounced dead at the 11-story building at East 63rd Street near York Avenue around 4:30 p.m. after landing on scaffolding below, cops said. Sources said it appeared to be a suicide.

“I heard the sound. It was so loud it woke me up,” said Brooke Bilven, 23, who lives across the street.

“It has shaken me up … It’s so sad to know somebody is that sad. I still can’t believe it.”

Bilven said that emergency workers tried to save the woman with CPR but were unsuccessful.

“I’m still shaken up. I still can’t get that sound out of my head,” said a witness who didn’t want to be named. “And to know it was a human being. It was so sad.”

It was unclear if the woman left a note.

Additional reporting by Antonio Antenucci