E-boat was the Western Allies' designation for the fast attack craft (German: Schnellboot, or S-Boot, meaning "fast boat") of the Kriegsmarine during World War II; E-boat could refer to a patrol craft from an armed motorboat to a large Torpedoboot. The name of E-boats was a British designation using the letter E for Enemy,

The main wartime production boats, the S-100 class, were very seaworthy, heavily armed and capable of sustaining 43.5 knots (80.6 km/h; 50.1 mph), briefly accelerating to 48 knots (89 km/h; 55 mph). These were armed with torpedoes and Flak guns; commonly one 37 mm at the stern, one 20 mm at the bow with a twin mount amidships, plus machine guns. Armament varied and some S-100s substituted a 40mm Bofors or, less commonly, a 20mm flakvierling mount for the aft 37mm cannon.

The S-100-class boats were 35 m (114 ft 10 in) long and 5.1 m (16 ft 9 in) in beam. Their diesel engines provided a range of 700 to 750 nmi (810–860 mi; 1,300–1,390 km), substantially greater than the gasoline-fueled American PT boats and British motor torpedo boats (MTBs).

As a result of early war experience of combat against the fast and powerful S-boats, the Royal Navy created its MGB force and later developed better-matched MTBs, using the Fairmile 'D' hull design.