Märket ('The Mark', Swedish pronunciation: [ˈmæ̂rːkɛt]) is a 3.3-hectare (8.2-acre) uninhabited skerry in the Baltic Sea shared by Sweden and Finland (in the area of the autonomous region Åland), with a lighthouse as its salient humanmade feature.

[1] Märket has been divided between the two countries since the Treaty of Fredrikshamn of 1809 defined the border between Sweden and Grand Duchy of Finland as going through the middle of the island.

The Finnish side of the island is part of the Municipality of Hammarland in the autonomous region of Åland and is the westernmost land point of Finland.

The route between Sweden and Åland has a passage about 15 nautical miles (28 km; 17 mi) long over open sea.

The halophilic grass Puccinellia capillaris and the herbaceous Sagina nodosa (knotted pearlwort) are found scattered throughout the island.

[9] The adjustment was carried out such that no net transfer of territory occurred, and the ownership of the coastline was unchanged so as not to interfere with each country's fishing rights.

The adjusted border takes the form of an inverted 'S', and the lighthouse is connected to the rest of Finland only by a short stretch of land.

[11] The border is marked by holes drilled into the rock, because the seasonal drift ice would shear off any protruding markers.

The Finnish part of Märket Reef used to be one of the world's most desired "countries" to contact among radio amateurs because of its special status and relative remoteness.

Amateurs with licenses in countries supporting CEPT can operate from the reef while using the OJ0/ prefix in front of one's own call sign.

[12] Märket has a continental climate affected by oceanic influences; it has a reputation for being one of the windiest places in Finland.