Pähkinäinen is an island located in Naantali. It can be accessed with a commuter ferry from Röölä in Rymättylä. There is a Turku City campsite in Pähkinäinen, and Turku owns the eastern half of the main island of Pähkinäinen. The western half of the island is circled by several kilometres of nature trail.
The majority of Pähkinäinen consists of rocky pine forests typical to the middle archipelago. Depressions with lush groves cut the long rock ridges here and there. A dense vegetation grows in these moist hollows.
The birdlife of Pähkinäinen is a compilation of species from the outer archipelago, herb-rich forests, and barren rocky pine forests. It is possible to spot a nutcracker (Nucifraga caryocatactes) which nests as a rarity in Finland only in the coastal zone.
The meadows of the area have an important role in the survival of insect species. Pähkinäinen is still one of the most important habitats of the Clouded Apollo (Parnassius mnemosyne), a butterfly which is protected in nature reserves.