Shinjuku Sumitomo Building, Skyscraper in Nishi-Shinjuku, Japan

The Shinjuku Sumitomo Building is a 52-story skyscraper in Nishi-Shinjuku defined by its distinctive triangular form with cut-off corners. A large central atrium dominates the interior, while a glass-covered public plaza called Sankaku Hiroba distinguishes the exterior.

The structure was completed in 1974 and stood as the tallest building in Japan and Asia at its opening. This marked a turning point in regional architecture and reflected Japan's economic strength during that era.

Locals call it "the Triangle" for its three-sided form with cut corners. At street level, the Sankaku Hiroba atrium hosts markets and events; before the 2017-2020 renovation the open plaza held local produce fairs and takigi noh firelit performances. A 1-metre water valve from the former Yodobashi Purification Plant, which occupied the site until 1965, is displayed as a memorial.

Toei Oedo Line Tochomae Station exit A6 opens directly at the building, while Shinjuku Station is about 8 minutes on foot and Marunouchi Line Nishi-Shinjuku Station about 6. Paid parking is available. The Sankaku Hiroba atrium, covered since the 2017–2020 renovation, doubles as a temporary shelter for 2,850 people during disasters.

The central atrium runs the full height of the building, so the floors form only a ring around a hollow core, a design compared to a bamboo tube for earthquake resistance. The visible emergency elevator on the exterior is used for cargo, not passengers. At ground level, a 1-metre water valve from the former Yodobashi waterworks (1898–1965) is displayed as a reminder of the site's earlier use.