Odakyu Dai-ichi Life Building, Office skyscraper in Nishi-Shinjuku, Japan

The Shinjuku Dai-ichi Seimei Building is a 115-metre skyscraper in Nishi-Shinjuku with a triangular floor plan: a central core holding the lifts, with rental offices on three sides. Its exterior is clad in sand-beige porcelain tiles. It forms a pair with the neighbouring Odakyu Century Building, the two set at 45 degrees around a shared axis with the large-roofed public open space Space Seven between them.

The building rose on plot 7 of the Shinjuku New Town, land bought jointly by Dai-ichi Life Insurance and Odakyu Electric Railway from the former Yodobashi water purification plant site. Dai-ichi Life made it its first high-rise investment building, while Odakyu built the neighbouring Odakyu Century Building. In September 2007 Odakyu's group acquired the trust beneficiary rights for over 30 billion yen, renamed the tower Odakyu Dai-ichi Life Building in April 2010, sold its stake to Dai-ichi Life and other domestic investors on 31 March 2023, and restored the Shinjuku Dai-ichi Seimei name in October 2023.

The building's name has changed twice: it became Odakyu Dai-ichi Life Building in April 2010, then reverted to Shinjuku Dai-ichi Seimei Building in October 2023. Odakyu Electric Railway's Shinjuku head office occupies part of it, and the ground floors hold tenants such as Ningyocho Imahan, a FamilyMart and a Pronto.

The building sits near multiple train stations and is within walking distance of convenient transit access. Its location in Nishi-Shinjuku makes it easy to reach and integrate into broader neighborhood exploration.

The building and its neighbour, the Odakyu Century Building, were built on plots 6 and 7 of the former Yodobashi water purification plant, bought jointly by Dai-ichi Life and Odakyu. The two towers share a triangular plan opening at 45 degrees around one axis, with the covered public plaza Space Seven between them.