SM U-6 or U-VI was a U-5-class submarine or U-boat built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K. Kriegsmarine) before and during the First World War. The submarine was built as part of a plan to evaluate foreign submarine designs, and was the second of three boats of the class built by Whitehead & Co. of Fiume after a design by Irishman John Philip Holland.