Universal suffrage or universal franchise refers to the right to vote being available for all adults within a jurisdiction. The idea is often endorsed by the slogan "one man, one vote".

For many, the term universal suffrage assumes the exclusion of the young and non-citizens (among others). At the same time, some insist that more inclusion is needed before suffrage can be truly universal. Democratic theorists, especially those hoping to achieve more universal suffrage, support presumptive inclusion, where the legal system would protect the voting rights of all subjects unless the government can clearly prove that disenfranchisement is necessary. Universal full suffrage includes both the right to vote, also called active suffrage, and the right to be elected, also called passive suffrage.

In the first modern democracies, governments restricted the vote to those with property and wealth, which almost always meant a minority of the male population. In some jurisdictions, other restrictions existed, such as requiring voters to practice a given religion. In all modern democracies, the number of people who could vote has increased progressively with time. The 19th century saw many movements advocating "universal [male] suffrage", most notably in Europe and North America. Female suffrage was largely ignored until the latter half of the century, when movements began to thrive; the first of these was in New Zealand, in which all adult women of all ethnicities gained the right to vote in 1893. A year later, South Australia granted all citizens the right to vote and stand for election, making it the first place in the world where women could stand as candidates for election to parliament. From there, this groundbreaking reform set a precedent for broader suffrage rights worldwide. However, voting rights were often limited to those of the dominant ethnicity.

In the United States, after the principle of "One person, one vote" was established in the early 1960s by the U.S. Supreme Court under Earl Warren, the U.S. Congress, together with the Warren Court, continued to protect and expand the voting rights of all Americans, especially African Americans, through the Civil Rights Act of 1964, Voting Rights Act of 1965 and several Supreme Court rulings. In addition, the term "suffrage" is also associated specifically with women's suffrage in the United States; a movement to extend the franchise to women began in the mid-19th century and culminated in 1920,^{[citation needed]} when the United States ratified the Nineteenth Amendment to the United States Constitution, guaranteeing the right of women to vote. It would be 1928 before voting rights were guaranteed to all women in the UK.

France, under the 1793 Jacobin constitution, was the first major country to enact suffrage for all adult males, though it was never formally used in practice (the constitution was immediately suspended before being implemented, and the subsequent election occurred in 1795 after the fall of the Jacobin government in 1794 discredited most ideas associated with them, including that constitution). Elsewhere in the Francophone world, the Republic of Haiti legislated for universal male suffrage in 1816. The Second French Republic instituted adult male suffrage after the revolution of 1848.

Following the French revolutions, movements in the Western world toward more universal suffrage occurred in the early 19th century, and focused on removing property requirements for voting. In 1867 Germany (the North German Confederation) enacted suffrage for all adult males. In the United States following the American Civil War, slaves were freed and granted rights of citizens, including suffrage for adult males (although several states established restrictions largely, though not completely, diminishing these rights). In the late-19th and early-20th centuries, the focus of the universal suffrage movement came to include the extension of the right to vote to women, as happened from the post-Civil War era in several Western states and during the 1890s in a number of British colonies.

On 19 September 1893 the British Governor of New Zealand, Lord Glasgow, gave assent to a new electoral act, which meant that New Zealand became the first British-controlled colony in which women had the right to vote in parliamentary elections. This was followed shortly after by the colony of South Australia in 1894, which was the second to allow women to vote, but the first colony to permit women to stand for election as well. In 1906, the autonomous Russian territory known as Grand Duchy of Finland (which became the Republic of Finland in 1917) became the first territory in the world to implement unrestricted universal suffrage, as women could stand as candidates, unlike in New Zealand, and without indigenous ethnic exclusion, like in Australia. It also led to the election of the world's first female members of parliament the following year. Federal states and colonial or autonomous territories prior to World War I have multiple examples of early introduction of universal suffrage. However, these legal changes were effected with the permission of the British, Russian or other government bodies, which were considered the sovereign nation at the time. For this reason, Australia (1901), New Zealand (1908) and Finland (1917) all have different dates of achieving independent nationhood.

The First French Republic adopted universal male suffrage briefly in 1792; it was one of the first national systems that abolished all property requirements as a prerequisite for allowing men to register and vote. Greece recognized full male suffrage in 1844. Spain recognized it in the Constitution of 1869 and France and Switzerland have continuously done so since the 1848 Revolution (for resident male citizens). Upon independence in the 19th century, several Latin-American countries and Liberia in Africa initially extended suffrage to all adult males, but subsequently restricted it based on property requirements. The German Empire implemented full male suffrage in 1871.