| Hiraoka Dam | |
| Dam Crosses: | Tenryū River |
| Location: | Tenryū, Nagano, Japan |
| Dam Type: | Concrete gravity dam |
| Dam Length: | 258 m |
| Dam Height: | 62.5 m |
| Dam Volume: | 252,000 m3 |
| Construction Began: | 1938 |
| Opening: | 1951 |
| Res Capacity Total: | 42,425,000 m3 |
| Res Catchment: | 3,650 km2 |
| Res Surface: | 258 ha |
is a dam in Tenryū, Nagano Prefecture, Japan, completed in 1951.[1] It is located on the Tenryū River upstream from the Sakuma Dam.
During the Second World War prisoners of war held at Tokyo #2D (Mitsushima) Prisoner of War Camp provided labour to build the penstock tunnel to the powerstation, collect aggregate, and mix cement for the construction of the dam between November 1942 and August 1945. Fifty seven prisoners died from diseases relating to poor sanitary, diet, and climate conditions.