The 'Bleeding Love' singer was signing copies of her autobiography Dreams at Waterstones in Piccadilly, London yesterday when the incident occurred.
Peter Kopwalczyk of South London has been charged with common assault for punching Lewis in the head, BBC News reports.
The 29-year-old was also sectioned under the Mental Health Act, according to a spokesperson for Scotland Yard. He is to appear at City of Westminster Magistrates' Court on October 26.
Kopwalczyk is said to have waited in line for five hours prior to the episode and was described as looking "like a nutcase" by an eyewitness.
A spokesperson for The X Factor winner said that she had been left "understandably shaken" following the attack. She later expressed her gratitude to fans for their supportive wishes.
Earlier today, the 24-year-old pulled out of a promotional tour of Germany due to the assault.