Samosir, or Pulau Samosir, is a large volcanic island in Lake Toba, which is located in the north of the island of Sumatra in Indonesia.

The lake and island were formed after the eruption of Mount Toba, a super volcano, approximately 70,000 years ago, in what was the last recorded supervolcanic explosion event. The island was originally connected to the surrounding caldera wall by a small isthmus, which was channeled through by the Dutch to aid in navigation on the lake.

At 630km², Samosir is also notable as being the largest island within an island, and the fourth largest lake island in the world. Samosir is a popular tourist destination due to the fantastic vistas it offers. The tourist resorts are concentrated in the Tuktuk area. The island considered to be the heart of the Batak culture and many of this people's artifacts remain on the island.