Rauma is about 240 kilometres northwest of Helsinki by road, roughly three hours on the E18 and Highway 8. From Turku it's an hour and a quarter, also on Highway 8.
There is no passenger railway to Rauma — the line was discontinued for passenger service decades ago, though freight still runs. Long-distance buses (Onnibus, Matkahuolto) connect Rauma with Helsinki, Turku, Tampere, and Pori several times daily; the bus station is a short walk from the old town.
The nearest airport is Pori (POR), 50 kilometres north, with a small number of domestic flights. Most international visitors fly into Helsinki-Vantaa or Turku and continue by bus or rental car. Rauma itself is walkable; you don't need a car in town.