Weh Island (Indonesian: Pulau Weh, Acehnese: Pulo Wèh) is a small active volcanic island to the northwest of Sumatra in Indonesia, also known as Sabang after the city situated on the northeast end of the island, whose area of 122.13 square kilometres (47.15 sq mi) includes the whole island as well as several offshore islets. It had a population of 43,527 at the official mid-2023 estimate. It is 45 minutes by fast regular ship or 2 hours by ferry from the mainland port of Banda Aceh. It was originally connected to the Sumatran mainland and became separated by sea after the stratovolcano's last eruption in the Pleistocene era. The island is situated in the Andaman Sea.

The island is known for its ecosystem; the Indonesian government has declared 60 square kilometres (23 sq mi) of inland and sea around the island as a wildlife protection area. A rare megamouth shark species was found on shore and the island is the only habitat for the threatened toad, Duttaphrynus valhallae (formerly Bufo valhallae). Coral reef areas around the island are known for their large variety of fish species.

Weh Island is located in the Andaman Sea, where two groups of islands, the Nicobar Islands and Andaman Islands, are scattered in one line from Sumatra to the north up to the Burma Plate. The Andaman Sea lies on an active moving small tectonic plate (microplate). A complex geological fault system and volcanic arc islands have been created along the length of the sea by the movement of the microplate.

The island lies about 15 kilometres (9 mi) off the northernmost tip of Sumatra. The island is small at only 121 km² (46.7 sq mi), but mountainous. The highest peak, Cot Kulam, is a fumarolic volcano, 617 metres (2,024 ft) high. The last known eruption is estimated to have occurred in the Pleistocene age, as a result which the mountain partially collapsed and was filled by the sea, forming a separate island.

At a depth of 9 metres (30 ft), close to Sabang city, underwater fumaroles emerge from the seabed. At Gapang Beach, one hour west of Balohan Port, there are also underwater fumaroles which are suitable for diving and are called Hydrothermal Point. A volcanic cone is found in the jungle. There are three solfatara (mudpot) fields on the island: one is 750 metres (0.5 mi) southeast of the summit and the others are 5 kilometres (3 mi) and 11.5 kilometres (7 mi) northwest of the summit, on the western shore of Lhok Perialakot bay.

There are four islets surrounding Weh Island: Klah, Rubiah, Seulako, and Rondo. Among those, Rubiah is well known for diving tourism, because of its coral reefs. When traveling to Saudi Arabia was only possible by sea, Rubiah was used as a place of quarantine for Indonesian Muslims during the Hajj pilgrimage season.

Weh Island is a part of Aceh province. A 1993 census reported 24,700 inhabitants. The large majority of the population are Acehnese and the remaining are Minangkabau, Javanese, Batak and Chinese. It is unknown when the island was first inhabited. Islam is the main religion, as Aceh is a special province where Sharia law has been applied exclusively for the province. However, there are some Christians and Buddhists on the island. They are mostly Javanese, Batak and Chinese.

On 26 December 2004, a 9.1–9.3 M_w undersea earthquake struck in the Andaman Sea. The earthquake triggered a series of tsunamis that killed at least 130,000 people in Indonesia alone. The effect on the island was relatively small, but it is unknown how many of its inhabitants were killed in the disaster.