Operation Sea Lion, also written as Operation Sealion (German: Unternehmen Seelöwe), was Nazi Germany's code name for the plan for an invasion of the United Kingdom during the Battle of Britain in the Second World War. Following the Battle of France, Adolf Hitler, the German Führer and Supreme Commander of the Armed Forces, hoped the British government would accept his offer to end the war, and he reluctantly considered invasion only as a last resort if all other options failed.

As a precondition, Hitler specified the achievement of both air and naval superiority over the English Channel and the proposed landing sites, but the German forces did not achieve either at any point during the war, and both the German High Command and Hitler himself had serious doubts about the prospects for success. Nevertheless, both the German Army and Navy undertook a major programme of preparations for an invasion: training troops, developing specialised weapons and equipment, and modifying transport vessels. A large number of river barges and transport ships were gathered together on the Channel coast, but with Luftwaffe aircraft losses increasing in the Battle of Britain and no sign that the Royal Air Force had been defeated, Hitler postponed Sea Lion indefinitely on 17 September 1940 and it was never put into action.