Radicalism (from French radical) was a political movement representing the leftward flank of liberalism during the late 18th and early 19th centuries and a precursor to social liberalism, social democracy, civil libertarianism and modern progressivism. This ideology is commonly referred to as "radicalism", but is sometimes referred to as radical liberalism or classical radicalism to distinguish it from radical politics. Its earliest beginnings are to be found during the English Civil War with the Levellers and later the Radical Whigs.

During the 19th century in the United Kingdom, continental Europe and Latin America, the term radical came to denote a progressive liberal ideology inspired by the French Revolution. Radicalism grew prominent during the 1830s in the United Kingdom with the Chartists and in Belgium with the Revolution of 1830, then across Europe in the 1840s–1850s during the Revolutions of 1848. In contrast to the social conservatism of existing liberal politics, radicalism sought political support for a radical reform of the electoral system to widen suffrage. It was also associated with a variety of ideologies and policies, such as liberalism, left-wing politics, republicanism, modernism, secular humanism, antimilitarism, civic nationalism, abolition of titles, rationalism, secularism, redistribution of property and freedom of the press.

In 19th-century France, radicalism was originally the extreme left of the day, in contrast to the social-conservative liberalism of Moderate Republicans and Orléanist monarchists and the anti-parliamentarianism of the Legitimists and Bonapartists. Until the end of the century, radicals were not organised as a united political party, but they had rather become a significant force in parliament. In 1901 they consolidated their efforts by forming the country's first major extra-parliamentary political party, the Republican, Radical and Radical-Socialist Party, which became the leading party of government during the second half of the French Third Republic (until 1940). The success of French Radicals encouraged radicals elsewhere to organize themselves into formal parties in a range of other countries in the late 19th and early 20th century, with radicals holding significant political office in Bulgaria (Radical Democratic Party), Denmark (Venstre), Germany (Progressive People's Party and German Democratic Party), Greece (New Party and Liberal Party), Italy (Republican Party, Radical Party, Social Democracy and Democratic Liberal Party), the Netherlands (Radical League and Free-thinking Democratic League), Portugal (Republican Party), Romania (National Liberal Party), Russia (Trudoviks), Serbia (People's Radical Party), Spain (Reformist Party, Radical Republican Party, Republican Action, Radical Socialist Republican Party and Republican Left), Sweden (Free-minded National Association, Liberal Party and Liberal People's Party) and Switzerland (Free Democratic Party). During the interwar period, European radical parties organized the Radical Entente, their own political international.

Before socialism emerged as a mainstream political ideology, radicalism represented the left-wing of liberalism and, thus, of the political spectrum. As social democracy came to dominate the centre-left and supplanted socialism, radicals either re-positioned as conservative liberals or joined forces with social democrats. Thus, European radical parties split (as in Denmark, where Venstre undertook a conservative-liberal rebranding, while Radikale Venstre maintained the radical tradition), took up a new orientation (as in France, where the Radical Party aligned with the centre-right, later causing the split of the Radical Party of the Left) or dissolved (as in Greece, where the heirs of Venizelism joined several parties). After World War II, European radicals were largely extinguished as a major political force except in Denmark, France, Italy (Radical Party) and the Netherlands (Democrats 66). Latin America still retains a distinct indigenous radical tradition, for instance in Argentina (Radical Civic Union) and Chile (Radical Party).