U-boats were naval submarines operated by Germany, particularly in the First and Second World Wars. Although at times they were efficient fleet weapons against enemy naval warships, they were most effectively used in an economic-warfare role (commerce raiding) and enforcing a naval blockade against enemy shipping. The primary targets of the U-boat campaigns in both wars were the merchant convoys bringing supplies from Canada and other parts of the British Empire, and from the United States, to the United Kingdom and (during the Second World War) to the Soviet Union and the Allied territories in the Mediterranean. German submarines also targeted Brazilian merchant ships during both World Wars and, twice over, precipitated Brazil's decision to give up its neutral stance and declare war on Germany.

The term is an anglicised version of the German word U-Boot [ˈuːboːt] (listen), a shortening of Unterseeboot (under-sea boat), though the German term refers to any submarine. Austro-Hungarian Navy submarines were also known as U-boats.