Nishi-no-shima (Japanese: 西之島, "western island") is a volcanic island located around 940 km (584 mi) south-southeast of Tokyo, that is part of the Volcano Islands arc. Nishinoshima is located about 130 km to the west of the nearest of the Ogasawara islands, hence the name, but the other Ogasawara island groups are aligned north-south. The nearest of the other Volcano islands is over 270 km away, but Nishinoshima is on the alignment of the Volcano islands. It was formed by ash from a underwater volcanic eruption.

Originally the above-water part of the ridge of an underwater caldera, Nishinoshima was enlarged in 1974 after fresh eruptions created a new section of the island. Another eruption that began in November 2013 further enlarged the island and attracted worldwide attention. A volcanic cone soon formed, rising to an estimated height of 142 m (466 ft) by July 2016. The eruptions ceased by November 2015, though emissions of volcanic gases continued for several months afterwards.

As of 2016, the island was about 2.7 km (1.0 sq mi) in size and showed evidence of the return of various plants and animal species.

There were further eruptions in 2017, 2018, and in 2019–2020. As of 14 August 2020, it is about 4.1 km (1.6 sq mi) and over 2 km (1.2 mi) in diameter.

The island was named "Rosario Island" when it was discovered by the Spanish ship Rosario in 1702. That was the island's name until 1904 when the Japanese term "Nishinoshima" (literally, "West Island") was made the official name.

When a new island was formed in the 1973 eruption, that was called Nishinoshima Shintō (西之島新島, literally, "New Nishinoshima Island"), but due to erosion and shifting sands, that island joined the main island and ceased to be considered a separate entity.

During the early stages of the 2013 eruption, a new volcanic island was formed southeast of the original Nishinoshima. The island was not given an official name but was mentioned in Japanese reports as "new island": atarashii shima (新しい島) or shintō (新島). Government officials said the island would be named "after it is stable and it is clear it will remain". As the island has now merged with Nishinoshima, "there is little possibility it will be named as a separate entity."

Prior to 1974, Nishinoshima was a small, green island which had no eruptions in the past 10,000 years. The island was merely the tip of an undersea volcano some 3,000 m (9,800 ft) in height and 30 km (19 mi) wide at the base. Its main period of activity was from the Late Pleistocene to the Holocene. The volcano takes the form of a caldera, with the original Nishinoshima and some nearby rocks forming part of the northwest ridge of a caldera about 1 km (0.6 mi) in diameter. The island was originally about 650 m (710 yd) long and 200 m (220 yd) wide. A 1911 survey determined the caldera was 107 m (351 ft) at its deepest. The volcano has many large, submarine, satellite cones to the south, west, and northeast. The southern cone rises to within 196 m (643 ft) of the surface, around 9 km (6 mi) SSE of Nishinoshima.

On 30 May 1973, the crew of a passing ship noted that, at around 11:00, white smoke rose east of the island to around 100 m (330 ft) in the air every few minutes. An aerial inspection the next day said that the eruption was taking place 400 m (440 yd) east of the island. A whirlpool was also seen. Yellow-green sea water and floating pumice were seen 5 km (3 mi) north of the site. Another crew discovered that two black rocks were rising from the sea.

In July, the eruption continued to erupt water fountains every few minutes with white smoke rising to 100 m above sea level. An aerial inspection on 14 September noted that a new island had formed above the submarine eruption site, the island formed a cinder cone around 40 m (130 ft) high and 120 m (130 yd) across. The new island had a 70 m (77 yd) diameter crater that frequently ejected cinders to a maximum height of 300 m (980 ft). White smoke rose to a height around 1,500 m (4,900 ft) high.

In November, a fissure eruption was noted on the new island with a chain of cinder cones running from southwest to northeast. The most recent cones were formed at the end of the chain, with the original cone being destroyed by the waves. The cones were erupting ash to a height of around 300 m (980 ft).

In December, the new island grew larger than "old" Nishinoshima. The island was 700 m (770 yd) long and 250 m (270 yd) wide; the new island had now developed a central crater that rose to a maximum height of 40 m (130 ft). The cone was continuing to eject ash and cinders to a height of 100 m (330 ft). A small craterlet on the new island was also emitting lava flows.

By February 1974, the new island had developed two cinder cones at the western end of the new island, but only the eastern cinder cone was erupting. Observations on 11 January noted that the active eastern cinder cone was emitting a lava flow.

In March, the new island was still erupting; by then, the island had five cinder cones; one cinder cone was erupting red hot lava. Lava flows were still erupting from the new island's flanks. On 1 March, the five cinder cones were quiet, but a large lava flow may have still been effusing from the island's flanks. After that, the eruption ended; wave movements then joined the new and old islands.