Peter Kowalczyk 29, allegedly punched Lewis, 24, in the head as she was signing copies of her autobiography, Dreams, at a bookstore in London's Piccadilly area. The south London resident will appear in court on Oct. 26.
"A guy came up and punched her to the side of the head," the pop star's spokesman told the BBC. A source described her as "okay" but "bruised."
On Thursday, Lewis, who didn't require hospitalization, wrote about the bizarre incident on her Web site.
"Yesterday was a horrible shock and left me extremely hurt and upset," she wrote. "I'm very sorry to those I wasn't able to meet at the signing."
Lewis had been scheduled to appear on a BBC program that night, but pulled out after the incident.