The Kungsbacka Fjord (Swedish: Kungsbackafjorden) is a fjord and a nature reserve in Halland County, western Sweden. The fjord is about 10 km long. It is delimited to the west by the Onsala Peninsula and to the east by the parish of Fjärås. The rivers Rolfsån and Kungsbackaån empty out into the fjord, which in turn opens to the Kattegat.