相模大堰, Water management facility in Ebina and Atsugi, Japan

Sagami weir (相模大堰) is a water control structure that crosses the Sagami River between the cities of Ebina and Atsugi, in Kanagawa Prefecture. It has several gates and channels that regulate the flow of the river and direct water toward the surrounding communities.

Construction began in 1995 as part of the Kanagawa Prefectural Waterworks Enterprise's Sagami River system project and was completed in 1998. The weir takes water released from the Miyagase Dam on the Nakatsu River and supplies it to Kanagawa Prefecture, Yokohama, Kawasaki and Yokosuka. Since March 31, 2020, its attached management bridge has been open to pedestrians and cyclists.

The weir's name, Sagami Ōzeki, simply combines the river's name with the word for a large weir. Its design was shaped by an environmental assessment: the rare plant Penthorum chinense was transplanted before construction, and the grounds include a biotope modelled on a wetland. The site is intended as an open place for community learning, with facilities for watching wild birds and fish climbing the fishways.

The weir lies 12 km upstream from the river mouth, with the left bank in Ebina and the right bank in Atsugi. The adjoining Sagami-ozeki management bridge opened to pedestrians and cyclists on 31 March 2020. The site is about 15 minutes on foot from JR Sagami Line Saka Station, along the Kanagawa Prefectural Route 409 Sagami River cycle path. The whole structure can be seen from the Tomei Expressway upstream.

The structure includes fish passages designed with different water speeds, so that both small and large fish can move through. These passages are shaped to match the natural movement patterns of the fish species living in the Sagami River.