The Thulin K was a Swedish naval fighter aircraft in the 1910s. It was operated by both the Swedish and Dutch armed forces.
Dr. Enoch Thulin, of AB Thulinverken, designed the Thulin K in December 1916. It was a shoulder-wing monoplane of wooden construction employing wing warping for lateral control. Powered by a 90 hp Thulin A Gnôme derived rotary engine, it could be configured as a single seat or tandem seat aircraft.
After initial flights in early 1917, the Swedish Army purchased two of the single seat K versions. However, it was more successful as an export plane; the Royal Netherlands Navy bought twelve K single seaters and three two seaters between 1917 and 1918. Both types were delivered without armament but the Dutch Navy fitted theirs with machine guns firing through the propeller arc and also experimented with 20 mm Madson cannons.
The aircraft's performance was good - Thulin himself broke the Swedish altitude record in it in 1919. However, on 14 May 1919 Thulin's own Thulin K lost an aileron in a steep dive, crashing and killing him. The rest of the K single seaters were written off in 1920 and the two seaters were withdrawn in 1922.