HMS G9 was a British G-class submarine built by Vickers, Barrow-in-Furness. She was laid down on 20 October 1915 and commissioned on 5 November 1916. During World War I, HMS G9 primarily operated in the North Sea.
The G-class submarines were designed as improved versions of the earlier E-class, with enhanced speed and diving capabilities. They also featured more powerful torpedoes. HMS G9 displaced 703 tons when surfaced and 837 tons when submerged. She was armed with 21-inch torpedo tubes, located in the bow, stern and amidships.
HMS G9 served throughout the war and was eventually sold on 20 January 1920 to Fred Watkins.