A federal Europe, also referred to as the United States of Europe (USE) or a European federation, is a hypothetical scenario of European integration leading to the formation of a sovereign superstate (similar to the United States), organised as a federation of the member countries of the European Union (EU), as contemplated by political scientists, politicians, geographers, historians, futurologists and fiction writers. At present, while the EU is not officially a federation or even a confederation, most contemporary scholars of federalism view the EU as a federal system, a supranational union, which has a flexible (see right to secession, Article 50 and Brexit) membership and competence delegation.

It is to be differentiated to a fused European State, or the concept of a European Republic, equalizing European regions, past the member states, as advocated by Ulrike Guérot.

Various versions of the concept have developed over the centuries, many of which are mutually incompatible (inclusion or exclusion of the United Kingdom, secular or religious union, etc.). Such proposals include those from Bohemian King George of Poděbrady in 1464; Duc de Sully of France in the seventeenth century; and the plan of William Penn, the Quaker founder of Pennsylvania, for the establishment of a "European Dyet, Parliament or Estates". George Washington also allegedly voiced support for a "United States of Europe", although the authenticity of this statement has been questioned.

Felix Markham notes how during a conversation on St. Helena, Napoleon Bonaparte remarked: "Europe thus divided into nationalities freely formed and free internally, peace between States would have become easier: the United States of Europe would become a possibility". "United States of Europe" was also the name of the concept presented by Wojciech Jastrzębowski in About eternal peace between the nations, published 31 May 1831. The project consisted of 77 articles. The envisioned United States of Europe was to be an international organisation rather than a superstate. Giuseppe Mazzini, an early advocate of a "United States of Europe" regarded European unification as a logical continuation of the unification of Italy. Mazzini created the Young Europe movement.

The term "United States of Europe" (French: États-Unis d'Europe) was used by Victor Hugo, including during a speech at the International Peace Congress held in Paris in 1849. Hugo favoured the creation of "a supreme, sovereign senate, which will be to Europe what parliament is to England" and said: "A day will come when all nations on our continent will form a European brotherhood ... A day will come when we shall see ... the United States of America and the United States of Europe face to face, reaching out for each other across the seas". During his exile from France, Hugo planted a tree in the grounds of his residence on the Island of Guernsey and was noted in saying that when this tree matured the United States of Europe would have come into being. This tree to this day is still growing in the gardens of Maison de Hauteville, St. Peter Port, Guernsey.

In 1867, Giuseppe Garibaldi and John Stuart Mill joined Victor Hugo at the first congress of the League of Peace and Freedom in Geneva. Here the anarchist Mikhail Bakunin stated: "That in order to achieve the triumph of liberty, justice and peace in the international relations of Europe, and to render civil war impossible among the various peoples who make up the European family, only a single course lies open: to constitute the United States of Europe". The French National Assembly also called for a United States of Europe on 1 March 1871.^{[citation needed]}

"Under a capitalist regime, the United States of Europe are either impossible or reactionary."

Before the communist revolution in Russia, Leon Trotsky foresaw a "Federated Republic of Europe — the United States of Europe", created by the proletariat. Following the First World War, some thinkers and visionaries again began to float the idea of a politically unified Europe. A Pan-European movement gained some momentum from the 1920s with the creation of the Paneuropean Union, based on Richard von Coudenhove-Kalergi's 1923 manifesto Paneuropa, which presented the idea of a unified European State. This movement, led by Coudenhove-Kalergi and subsequently by Otto von Habsburg, is the oldest European unification movement. In 1923, the Austrian Count Richard von Coudenhove-Kalergi founded the Pan-Europa Movement and hosted the First Paneuropean Congress, held in Vienna in 1926. The aim was for a Europe based on the principles of liberalism, Christianity and social responsibility.