The Hansa-Brandenburg W.18 was a single-seat biplane fighter flying boat designed by the Hansa-Brandenburg Aircraft Company () for the Imperial German Navy's () Naval Air Service () and the Austro-Hungarian Navy's () () during the First World War. It was developed from the Hansa-Brandenburg CC in 1916.
The prototype was delivered to the Naval Air Service with the majority purchased by Austro-Hungarian Naval Aviation. Hansa-Brandenburg built 47 for them and also sold a license to Phönix Flugzeug-Werke to build an additional 60.
The pusher configuration W.18 was derived from the earlier CC fighter and shared that aircraft's single-bay wings, swept lower wing, wing floats, and a single-step hull to allow the aircraft to break free from the water more easily. It replaced the latter's star-shaped interplane struts with two vertical struts on each side. The aircraft built for the Naval Air Service retained the CC's water-cooled Benz Bz.III straight-six engine that was mounted on struts between the wings. The armament consisted of two fixed, forward-firing LMG 08/15 machine guns.
Forty-seven aircraft were ordered by Naval Aviation on 14 December 1916, modified to suit Austro-Hungarian preferences. The fuselage was lengthened to improve stability and minor changes were made to the tail structure, wings and the LMG 08/15 machine guns were exchanged for Schwarzlose M-16 machine guns. The Benz engine was replaced by a water-cooled Hiero 6 straight-six engine with the radiator embedded in the upper wing. The aircraft license-built by Phönix were further modified and only had a single Schwarzlose machine gun.
The prototype was delivered to the Naval Air Service's Seaplane Experimental Command () in December 1917, but was received with little interest by the Germans. Deliveries of all 47 aircraft were completed by February 1918, although the last 9 were delivered without engines.
Austro-Hungary received 47 W.18s, using them to provide air-defence for ports and naval bases along the Adriatic Sea coast.