There is no airport in Sifnos island. The nearest airport with national flights is located in the neighboring island of Milos and receives frequent flights from Athens in summer. The closest international airport is on Santorini island.
From the port of Milos, there are regular ferries to Sifnos island and other Greek islands. The ferry trip from Milos to Sifnos is about 1-2 hours, depending on the ferry type.
To get a better understanding of the island's location, view the Map of Cyclades.