The ship, which belongs to P&O Ferries, was crossing the Channel from Calais in France to Dover in Britain when the unidentified passenger fell into the sea, prompting a five-hour search operation, the French National Sea Rescue Society said.
According to P&O Ferries, the "Pride of Kent" ferry was just under 10 kilometres (six miles) away from the English coast when a passenger alerted the crew that another person had gone overboard.
The company said it could not yet determine whether the fall was accidental or intentional.
Four British and French lifeboats, a Royal Air Force helicopter and another P&O ferry launched a search for the passenger between 0200 GMT and 0700 GMT Thursday, to no avail.
Search operations have been suspended for now.