Storeim

Østerøya (lit. 'East Island') a peninsula in Sandefjord Municipality in Vestfold county, Norway. It is one of two long, narrow peninsulas located south of the city of Sandefjord. The peninsula lies between the Mefjorden and Tønsbergfjorden and it sits across the fjord from the Vesterøya peninsula. The Tønsberg Barrel is located on the southern end of the peninsula. The Tønsberg Barrel is an old sea mark that is mentioned in Sverris saga. It has been described as one of the most beautiful sites in Sandefjord.^{: 56, 57, 297}

Østerøya has a large number of beaches, campgrounds, recreational areas, and vacation homes. The peninsula has a length of 10 kilometres (6.2 mi), and is between 1–2 kilometres (0.62–1.24 mi) in width. It was previously known as Yxnøy, a name which now is used for the southernmost point of the peninsula. Yxnøy, also spelled Yxney, is now a recreational area home to several beaches, forests, sloping rocks, hiking trails, and Tønsberg Barrel.^: 101 The peninsula is now named Østerøya ("east island") since it is located east of the centre of the city of Sandefjord. The southern end of the peninsula contains public areas for outdoor activities, recreation, fishing and swimming. A preserved area with black alder swamp forest (containing ramsons, iris pseudacorus and early purple orchid) is located here.

During the Viking Age, ocean water levels were about 3 metres (9.8 ft) higher, which meant both Vesterøya and Østerøya peninsulas were islands surrounded by ocean waters.

There are 20 kilometres (12 mi) of hiking trails on Østerøya, which is an extension of the 25 kilometres (16 mi) coastal path at Vesterøya. The combined hiking trail is part of the international North Sea Trail.

Head Above Water ("Hodet over vannet") by film director Nils Gaup was filmed at the southern tip of Østerøya and features Ertsvika at Yxnøy.^: 74