Alonnisos is the third largest island of the N. Sporades and the only one inhabited (2,800 people) within the National Marine Park of Alonnisos, Northern Sporades. The main port of Alonnisos is Patitiri, while the part of the island that has kept its traditional local architectur is the Old Town.

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Alonnisos is covered by high bushes and trees, typical of the Mediterranean flora, such as arbutuses and yews in the north, pine trees in the south, and small bushes scattered around the rest of the island. An important habitat is the only wetland on the island, in Aghios Dimitrios.

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All around the island one can find a large number of beautiful beaches with clear crystal waters, some of which are well organized, while others remain untouched. Since 2011, scuba diving is allowed at 14 points within the Marine Park. The pristine seabed, the rich flora and fauna, along with the numerous ancient wrecks, make Alonnisos the ideal place for scuba diving aficionados. For those who are more in favour of land-based activities, Alonnisos has many trekking paths to explore the island's natural beauty.