Alonnisos (Greek: Αλόννησος), also transliterated as Alonnisos or Alonisos, is a Greek island in the Aegean Sea. After Skiathos and Skopelos it is the third member of the Northern Sporades. It is 3 km (2 nm) E of the island of Skopelos. Alonnisos is also the name of a village on the island, as well as the municipality that encompasses the island and the village.

The village of Alonnisos is located on the southern part of the island. It is locally known as Chora. The main port of the island is located in the southeast and is called Patitiri. There is ferry and hydrofoil service from Patitiri to Volos, Agios Konstantinos, and Thessaloniki on the mainland and to the islands of Skiathos, Skopelos and Skyros. The bay at the southern end of the island is also called Alonnisos.

The Municipality of Alonnisos includes the nearby islands of Adelfoi, Gioura, Kyra Panagia (Pelagos), Peristera, Piperi, Psathoura and Skantzoura.

In the Middle Ages and until the 19th century, the island was known as Liadromia (Λιαδρόμια). It was renamed in 1838, as it was – mistakenly according to later research – identified with Alonnisos of Antiquity. In reality, the present island of Alonissos was known as Ikos (Ίκος) to the Ancient Greeks.

The island at its widest is 4.5 km from northwest to southeast and at its longest is 20 km from southwest to northeast. The area of the island is 64 km2. The island is mostly limestone. It is located east of mainland Greece and Magnesia, northeast of Euboea and northwest of the island of Skyros.

Agriculture is widespread on Alonnisos, predominantly mixed farming and vineyards. The main products are almonds, grapes, figs and olives. Spruce trees are abundant on the island. While fishing is a popular profession on the island, tourism is now the main industry.

Housing traditionally consists of closely constructed houses with stone walls, which, until 1965, were centered around the capital. A small fortress protected against enemies and pirates. The island and the village were heavily damaged in the 1965 earthquake. Many inhabitants returned not to the village of Alonnisos, but to Patitiri. The old village has been restored in recent years, with rebuilt houses now utilised for tourism.

The beaches around the island consist of pebbles. The ecosystem in the north side of the island is in good condition. The Mediterranean Monk Seal (Monachus-Monachus) is common around the island, and in 1992 the Alonnisos Marine Park was created to protect these seals and other animals.