The PT-9-class, PT-9 (PT-809) was a British Dark-class fast patrol boat purchased by the JMSDF in 1954 as a reference vessel for evaluating fast-patrol technology and composite construction methods. Built by Saunders-Roe in the United Kingdom, it was the only Dark-class boat operated by Japan and was commissioned in September 1957. Powered by two Napier Deltic diesel engines, it could reach 40 knots and was armed with a single 40 mm Bofors gun and two 53 cm torpedo tubes. PT-809 was extensively used for technical trials and provided valuable data for the development of subsequent Japanese torpedo boats, specifically the PT-11-class, before being retired in 1972.

The PT-809 was introduced in Update 2.59 "Sky Odyssey". As a technology demonstrator and test bed, the Japanese configuration retained the Dark-class's high-speed characteristics while carrying only two 53 cm torpedo tubes instead of the four-tube maximum, giving it exceptional mobility at the expense of torpedo capacity. Its single 40 mm Bofors cannon is mounted at the bow, providing reliable firepower against smaller vessels but leaving the PT-809 heavily reliant on its speed and torpedoes when engaging larger targets. Its high top speed allows it to rapidly capture objectives, reposition, and disengage from unfavourable engagements, although its limited armament and survivability make prolonged engagements risky.