Kirui Island or Kirui Island Marine Reserve is a 36.095 km² protected, uninhabited island in the Pemba Channel under the Tanga Marine Reserves (TMRS) with the IUCN category II located within Mkinga District in Tanga. The Island is the largest in Tanga and one of the largest protected marine Islands in Tanzania Mangroves are thought to cover 680 ha (ha) of the island of Kirui, with the largest mangrove area (557 ha) being on the north also the Tanzania‐Kenya border and west sides, Kigomeni mangrove (60 ha) being on the east coast, and Kirui South Mangrove (63 ha) being on the south.

Kirui Island's eastern side is completely covered in coral reefs. In general, seagrass beds rule Moa Bay, with a patchy coral reef in the middle of the bay, where it is deeper. At Ras Gomani, a location outside the borders of the Kirua Island Marne Reserve, excellent coral reefs were seen.

Historically Kirui Island was first inhabited by Wadiko, but most have now moved to Kijiru, and Ndumbani villages in Moa Ward, Mkinga District. Settlement in the Island was divided in two areas: Kirui at the southern part, Kendwa and Mbayai on the northern part. More than 20 families lived in Kirui Island. They started to move out in early 1960s due to increased death rate, which was linked some kind devils.  On their route to the Jasini area, some migrated to Kiphururwe, while others went to Kijiru and Ndumbani. They continue to travel to Kirui and Mbayai to honour their ancestors.