A security perimeter was set up on a P&O ferry on Wednesday morning after crew discovered an unattended package on board.
Local authorities said the package was a metal box wrapped in tape, reported the La Voix du Nord newspaper.
After an hour and a half, the package was deemed not to be dangerous.
Ferry traffic was not interrupted during the bomb squad's work, and no passengers were on board the ship at the time.
France remains on edge as the state of emergency continues, over a year since it was set up on the day of the November terror attacks in 2015.