HMS Mayfly may refer to two vessels of the British Royal Navy named after the mayfly:

• Mayfly, a torpedo boat launched in January 1907, having been named HMS TB 11 whilst under construction; sunk by a naval mine in March 1916
• HMS Mayfly (1915), a Fly-class river gunboat constructed in sections at Yarrow in 1915; lent to the War Department in January 1918; sold at Basra in March 1923