The Belgian Government in London (Dutch: Belgische regering in Londen; French: Gouvernement belge à Londres), also known as the Pierlot IV Government, was the government in exile of Belgium between October 1940 and September 1944 during World War II. The government was tripartite, involving ministers from the Catholic, Liberal and Labour Parties. After the invasion of Belgium by Nazi Germany in May 1940, the Belgian government, under Prime Minister Hubert Pierlot, fled first to Bordeaux in France and then to London, where it established itself as the only legitimate representation of Belgium to the Allies.

Despite no longer having authority in its own country, the government administered the Belgian Congo and held negotiations with other Allied powers about post-war reconstruction. Agreements made by the government in exile during the war included the foundation of the Benelux Customs Union and Belgium's admission into the United Nations. The government also exercised influence within the Belgian army-in-exile and attempted to maintain links with the underground resistance.

Politically, Belgian politics had been dominated in the interwar period by the Catholic Party, usually in coalition with the Belgian Labour Party (POB-BWP) or the Liberal Party. The 1930s also saw the rise in popularity of Fascist parties within Belgium; most notably Rex which peaked at the 1936 election with 11% of the vote. From the early 1930s, Belgian foreign and domestic policy had been dominated by the policy of neutrality; leaving international treaties and alliances and attempting to maintain good diplomatic relations with Britain, France and Germany.

Despite this policy, Belgium was invaded without warning by German forces on 10 May 1940. After 18 days of fighting, the Belgian military surrendered on 28 May and the country was placed under the control of a German military government. Between 600,000 and 650,000 Belgian men (nearly 20% of the country's male population) had been mobilized to fight.

Unlike the Netherlands or Luxembourg, whose monarchies went into exile alongside the government, King Leopold III, whose mother was German, surrendered to the Germans alongside his army – contrary to the advice of his government. In the days before his surrender, he allegedly attempted to form a new government under the pro-Nazi socialist Henri de Man though this was never realized. He remained a prisoner of the Germans, under house arrest, for the rest of the war. Although the government briefly attempted to negotiate with the German authorities from exile in France, the German authorities passed a decree forbidding members of the Belgian government returning to the country and the talks were abandoned.

The Belgian government in France had been intending to follow the French government of Paul Reynaud to France's overseas empire to continue the fight. Meanwhile, Germany appointed General Alexander von Falkenhausen, an aristocrat and career soldier, as military governor of Belgium. The government was briefly established in Limoges where, under pressure from the French government, they denounced Leopold's surrender. However, when Reynaud was replaced by the pro-German Philippe Pétain, this plan was abandoned. Despite hostility from the new Vichy regime, the Pierlot government remained in France. On 16 September 1940, Vichy demanded the disbandment of the Belgian government, still at that time in Bordeaux:

The Belgian government, whose activity in France has been, for some time now, purely theoretical, will decide to dissolve itself. Some of its members will remain in France as private individuals, while others will go abroad. This decision is part of the suppression of diplomatic missions of countries occupied by Germany, the necessity of which has been pointed out to the French government by the Reich.

— Letter from the Vichy French government, 16 September 1940.