Pserimos is a small island connected by sea to the nearby islands of Kalymnos and Kos, both reachable in about 40 minutes by boat or just 10 minutes by speedboat. The boat from Kalymnos is called Maniai. Tickets cannot be booked online.
The easiest way to reach Pserimos is to fly to Kos (connected to Athens and many European cities) and take the short boat crossing.
Connections are more frequent during the summer season; always check availability if travelling in other periods.