You will find taxis waiting out of the port, 24/7. You will get to the city centre in about 30 minutes.
The fare from Piraeus port to Athens centre (Syntagma), including pretty much everything (luggage charge, tolls fee, VAT, meter start, airport charge, etc.), will cost you approximately:
– Day time (05:00-24:00): 25 €
– Night time (24:00-05:00): 40 €
The same fares should apply when you pick a taxi from Athens centre to go to Piraeus port.
Be prepared to pay cash, as credit cards are not accepted in Greek taxis.
Tip: A great alternative is Beat, the Greek popular taxi app. Download Beat app, select your driver from the list of user-rated drivers’ profiles, pay cash or via credit card/paypal online and rate your driver.