The Sōryū-class submarines (16SS) are diesel-electric attack submarines that 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 tail planes. The Sōryūs have the largest displacement of any submarine used by post-war Japan.
It is Japan's first AIP submarine. The class are fitted with air-independent propulsion based on Kockums stirling engines license-built by Kawasaki Heavy Industries, allowing them to stay submerged for longer periods of time. Some recent information suggest that the last units in the series will have their air independent propulsion systems replaced with banks of Li-Ion batteries.