All 875 passengers were ordered off a Greek ferry at the port of Piraeus on Monday after a bomb threat.
A search found no explosives, the coast guard said.
The alert came shortly before the Blue Star 1 was due to leave at 6 p.m. for the Aegean islands of Syros, Patmos, Leros, Kos and Rhodes.
An unknown caller phoned a passenger and warned that a bomb on board was about to explode.
The passenger alerted the crew, who notified port authorities.
Coast guard bomb experts and a sniffer dog searched the vessel, which also carried 83 crew members and more than 200 vehicles, including 110 trucks.
They found nothing suspicious.
Passengers were allowed to board again, and the ferry left on its scheduled route.
The caller has not been identified, and no motive has been reported.
Authorities are investigating.
Piraeus is Europe's busiest passenger port, and ferries are the main link between the Greek mainland and its islands.