永井寺, Buddhist temple in Koga, Japan.

Nagai Temple is a Buddhist temple in Koga featuring wooden construction with curved roofs and traditional bracket systems supporting multiple building levels. The structures display classic Japanese carpentry techniques with exposed posts and cross-beams that hold up the intricate roof framework.

Nagai Naokatsu, lord of Koga Castle, founded the temple in 1626 as the family's burial temple; his son Naomasa built it for his father's memorial rites. It stood in samurai quarters near the castle's Otte-mon gate, beside the moat of the Kannon-ji enclosure. The Nagai family graves, including Naokatsu's tomb and a stele inscribed by Hayashi Razan, are a city-designated historic site.

The temple is the family temple (bodaiji) of the Nagai clan, founded in 1626 by Nagai Naokatsu, lord of Koga Castle, and his son Naomasa. The Nagai family cemetery here holds Naokatsu's grave with a hōkyōintō tower and a memorial stele inscribed by the Confucian scholar Hayashi Razan. The temple also contains the grave of the calligrapher Koyama Kaigai, moved here in 1949.

The temple lies in Nishimachi, Koga, about 17 minutes on foot from the west exit of JR Koga Station (roughly 1.4 km) and about 18 minutes from the east exit of Tobu Nikko Line Shin-Koga Station (roughly 1.5 km). The city's free rental bicycle service, Kogattsu, can be used for the trip. The Nagai family graves, the grave of calligrapher Koyama Kagai and the grave of Kodaka Ekikei stand within the grounds.

A writing desk kept at the temple is said to have been made from the door of the former Koga Castle three-story keep when it was dismantled. The desk is a lesser-known object linking the temple to the castle that once stood nearby.