The popular comic has made few public appearances since reportedly walking away several years ago from a $50 million contract to continue starring in his sketch comedy series "Chappelle's Show."
The Oregonian said thousands of curious fans showed up at the square around midnight in hopes of seeing Chappelle perform his stand-up act. The comedian arrived shortly before 1 a.m. and addressed the audience, which greeted him with thunderous applause. However, few reportedly could hear what he said because there was no adequate sound system.
Chappelle seemed to be without an entourage or security, and it was unclear why he was in town or when he decided he wanted to meet the locals.
"I'm not here for money," he quipped, adding he is happy "when people can stand this close together and not be angry."
"This has never happened in my entire career," Chappelle said, indicating the crowd was much bigger than he expected. "I'm grateful everybody came out."
Portland Police spokeswoman Mary Wheat told The Oregonian the comedian didn't have a permit, but she emphasized there was no trouble at the impromptu event.
"This is definitely a social networking town," she said. "Everything went fine."
"Portland Police were kind enough not to arrest me," Chappelle said during his appearance, which lasted about an hour.
As he wrapped up his appearance he joked about the lesson he learned during his visit -- "Don't tell a secret in Portland."