South Harbour is a bay and harbour area immediately next to the centre of the city of Helsinki, Finland. It is the largest passenger harbour in Finland, and 5.4 million passengers travel through it every year. The most of the harbour's traffic is to Stockholm, Sweden and Tallinn, Estonia, and cruises. In summertime, there is also much small ship traffic.
The bay is bordered by the districts of Katajanokka, Kaartinkaupunki, Ullanlinna and Kaivopuisto. The waterway leading to the South Harbour is 9.6 metres deep. The most critical point on the waterway is the Kustaanmiekka strait, with a width of 80 metres. The waterway has a speed limit of 30 km/h, except for the Katajanokka area, which has a speed limit of 10 km/h.