Ferries travel from Piraeus to Mytilene everyday, and sometimes twice per day (8 or 12 hours voyage, depending on the ferry). The companies that provide ferries for Lesvos are NEL LINES, ANEK LINES, HELLENIC SEAWAYS.

In order to learn the exact schedules of the ferries you can also contact the Mytilene's port authorities,
Tel (+30)2251040827,  (+30)2251024115.
Here you can find useful information about the daily schedules from Piraeus port and also how you can get there.