Unrestricted submarine warfare is a type of naval warfare in which submarines sink merchant ships such as freighters and tankers without warning. The use of unrestricted submarine warfare has had significant impacts on international relations in regard to both the First World War and the Second World War. Its history has been dominated by German decision making.

There have been attempts to limit the use of unrestricted naval warfare, with some dating back to before the turn of the 20th century as an extension of rules for surface raiders. While initially submarines operated successfully by attacking on the surface using deck guns, attacking without warning while submerged reduces the opportunity for the target to escape or defend itself if armed.

Customary naval law (specifically, so called cruiser rules) specified that while enemy warships may be attacked freely, civilian and neutral ships can only be interfered with if carrying contraband (announced previously in a contraband list), and the lives of the crew should be protected. Formal limitations on warfare at sea date back to the 1899 Hague Convention.

However, the Imperial German navy was heavily criticised internally by high level officials for their relative inactivity at the start of WWI. To boost the role of the navy, and buoyed by early successes of U-boat warfare, Admiral Tirpitz and Admiral von Pohl suggested a plan whereby U-boats, given a free hand to attack shipping, could potentially force Britain into a "concillatory mood" in as few as six weeks. The admirals appealed to public opinion through press interviews, posing the submarines as "miracle weapons", despite the extremely small number of vessels available. It was believed that a "shock effect" would cause shipping to cease, and that neutrals would judge the campaign a reasonable reprisal for the British naval blockade. Chancellor Bethmann Hollweg accepted this strategy on February 1, 1915, and a directive issued the next day, with a public announcement on the 4th.

This first campaign was not fully unrestricted as it was aimed at Allied vessels, with neutral shipping officially not to be targeted. Many submarine commanders also chose to adhere to cruiser rules anyway. However, the German Admiralty encouraged the U-boats to attack without warning and minimise efforts at identifying targets, as "accidental" sinking of neutral vessels was viewed to have a useful deterrent effect. In the end, the German campaign did not have a significant impact on Britain's goods traffic, but took a heavy civilian toll, including to neutrals. In the most dramatic episode they sank Lusitania in May 1915 in a few minutes, killing over a hundred American passengers. In the face of US anger, German Chancellor Bethmann Hollweg obtained a secret directive to exclude passenger vessels from being targeted and to make strenuous measures to avoid striking neutral vessels, a measure made into a formal and public suspension of unrestricted warfare after the sinking of Arabic in August 1915. Submarines operated under prize rules for 1916 - indeed even during 1915 the majority of attacks were made on the surface.

Admiral Henning von Holtzendorff, chief of the Imperial Admiralty staff, argued successfully in early 1917 to resume unrestricted attacks, at a greater scale than 1915 and thus hopefully successfully starve the British into surrender. The German high command realized the resumption of unrestricted submarine warfare meant war with the United States but calculated that American mobilization would be too slow to stop a German victory on the Western Front. The decision made by Germany became one of the "trigger mechanisms" causing the United States, who were previously neutral, to join the war in favour of the British. While initially successful, the U-boats would once again fall short of the hopes of the German Admiralty.

After World War I, there was a strong push to construct international rules prohibiting submarine attacks on merchant ships. In 1922 the United States, the United Kingdom, Japan, France and Italy signed the Washington Treaty on Poison Gas and Submarines, to so restrict the use of submarines as to make them useless as commerce raiders. France did not ratify, so the treaty did not go into effect.

In 1936, states signed the London Protocol on Submarine Warfare. To be deemed acceptable, naval attacks needed to follow prize rules, which called for warships to search merchantmen and place crews in "a place of safety" before sinking them.