Nii-jima (新島^?) is a volcanic Japanese island in the Philippine Sea, administered by Tokyo Metropolitan Government. It is one of the Izu Seven Islands group of the seven northern islands of the Izu archipelago, and is located approximately 163 kilometres (101 mi) south of Tōkyō and 36 kilometres (22 mi) south of Shimoda Shizuoka Prefecture. The island is the larger inhabited component of the village of Niijima Village, Ōshima Subprefecture of Tokyo Metropolis, which also contains the larger, neighboring island of Shikine-jima and the smaller, uninhabited Jinai-tō. Nii-jima is also within the boundaries of the Fuji-Hakone-Izu National Park.

On the southern tip of Nii-jima, was formerly a launch site for experimental and sounding rockets.

Nii-jima is unusual amongst the Izu Islands in that it has an elongated shape. Measuring approximately 11 kilometres (6.8 mi) long by 2.5 kilometres (1.6 mi) wide, it has a land area of 23.87 km². The island is made of eight rhyolitic lava domes in two groups at the northern and southern ends of the island, separated by a low, flat isthmus. The Mukai-yama (向山^?) complex in the southern portion of the island and the Achiyama lava dome at the northern end were formed during Nii-jima's only historical eruptions in the 9th century AD. The northern end also contains Miyatsuka-yama (宮塚山^?), the island’s highest point, at 432 metres (1,417 ft). Shikine-jima and Jinnai-to are part of the same complex, and form separate islands to the southwest and west of Nii-jima. Rhyolite lava gives the island its famed white cliffs and white sandy beaches