Matiu/Somes Island, at 24.9 ha (62 acres), is the largest of three islands in the northern half of Wellington Harbour, New Zealand. It lies 3 km (1.9 mi) south of the suburb of Petone and the mouth of the Hutt River, and about 5 km (3.1 mi) northwest of the much smaller Makaro/Ward Island.

Legend has it that both Matiu and Makaro Islands received their original Mâori names from Kupe, the semi-legendary first navigator to reach New Zealand and get home again with reports of the new land. He named them after his two daughters (or, in some versions of the tale, nieces) when he first entered the harbour about 1000 years ago.

The island has had an extensive Mâori history and a varied, and a sometimes colourful and tragic, European one. Prior to the mid 19th century there were 2 Mâori Pa on the island, however, like the Ngati Ira Pa on Ward Island, they were not permanently inhabited, being "Pa of refuge" where the tribespeople could retreat to in times of war. One was in the centre of the island and little more than the remains of some middens are left there, however there was another Pa on the northern tip of the island, strategically positioned with cliffs on three sides for ease of defence.

The more recent history of the island has been most eventful. A lighthouse was built at the island's southwestern end in 1866, the first harbour light in New Zealand. The lighthouse that stands in place of the original today was constructed in 1900 and later automated. At various points throughout the 20th century it hosted enemy alien internees during wartime, and quarantine facilities for both human immigrants and animals. In 1942, the island was fortified with heavy anti-aircraft gun emplacements on the summit, but these were never used during the course of the war. This whole area was levelled flat for the purpose of this construction, with the result that 17 metres was removed from the island's previous overall height. A degaussing station was built to provide protection for ships against magnetic mines. Many of the physical features of these sites are present on the island today.

Matiu/Somes became part of Lower Hutt in 1989, and came under the full control of the Department of Conservation ("DOC") as a scientific and historic reserve in August 1995. The island is free of introduced mammalian predators (such as stoats), an unusual state for an island so close to an urban centre.