The Sōryū-class submarines (16SS) are diesel-electric attack submarines. The first boat in the class entered service with the Japan Maritime Self-Defense Force in 2009. The design is an evolution of the Oyashio-class submarine, from which it can most easily be distinguished by its X-shaped stern combination diving planes and rudders. The Sōryūs have the largest displacement of any submarine used by post-war Japan.
It is Japan's first air-independent propulsion submarine. From Sōryū to Shōryū are fitted with Kockums Naval Solutions Stirling engines license-built by Kawasaki Heavy Industries, allowing them to stay submerged for longer periods of time. Furthermore, Ōryū is the world's first lithium-ion battery submarine. The cost of the sixth submarine (Kokuryū) was estimated at 540 million USD.
In 2019, the replacement to the Sōryūs, the Taigei-class submarine,[5] entered the planning phase.
