State socialism is a political and economic ideology within the socialist movement that advocates state ownership of the means of production. This is intended either as a temporary measure, or as a characteristic of socialism in the transition from the capitalist to the socialist mode of production or to a communist society. State socialism was first theorised by Ferdinand Lassalle. It advocates a planned economy controlled by the state in which all industries and natural resources are state-owned.

Aside from anarchists and other libertarian socialists, there was, in the past, confidence amongst socialists in the concept of state socialism as being the most effective form of socialism. Some early social democrats in the late 19th century and early 20th century, such as the Fabians, claimed that British society was already mostly socialist and that the economy was significantly socialist through government-run enterprises created by conservative and liberal governments which could be run for the interests of the people through their representatives' influence, an argument reinvoked by some socialists in post-war Britain. State socialism declined starting in the 1970s, with stagflation during the 1970s energy crisis, the rise of neoliberalism and later with the fall of state socialist nations in the Eastern Bloc during the Revolutions of 1989 and the fall of the Soviet Union.

Libertarian socialists often treat state socialism as synonymous with state capitalism, arguing that the economic systems of Marxist–Leninist states such as the Soviet Union were not genuinely socialist due to their continued use of commodity production, and wages as remuneration. Democratic and libertarian socialists claim that these states had only a limited number of socialist characteristics. However, others maintain that workers in the Soviet Union and other Marxist–Leninist states had genuine control over the means of production through institutions such as trade unions. Academics, political commentators and other scholars tend to distinguish between authoritarian state socialism and democratic state socialism, with the first representing the Soviet Bloc and the latter representing Western Bloc countries which have been democratically governed by socialist parties such as Britain, France, Sweden and Western social-democracies in general, among others.

As a classification within the socialist movement, state socialism is held in contrast with libertarian socialism, which rejects the view that socialism can be constructed using existing state institutions or governmental policies. By contrast, proponents of state socialism claim that the state—through practical governing considerations—must play at least a temporary part in building socialism. It is possible to conceive of a democratic socialist state that owns the means of production and is internally organised in a participatory, cooperative fashion, thereby achieving both social ownership of productive property and workplace democracy. Today, state socialism is mainly advocated by Marxist–Leninists.

The role of the state in socialism has divided the socialist movement. The philosophy of state socialism was first explicitly expounded by Ferdinand Lassalle. In contrast to Karl Marx's perspective, Lassalle rejected the concept of the state as a class-based power structure whose primary function was to preserve existing class structures. Lassalle also rejected the Marxist view that the state was destined to "wither away". Lassalle considered the state an entity independent of class allegiances and an instrument of justice that would therefore be essential for achieving socialism.

Early concepts of state socialism were articulated by anarchist and libertarian philosophers who opposed the concept of the state. In Statism and Anarchy, Mikhail Bakunin identified a statist tendency within the Marxist movement, which he contrasted to libertarian socialism and attributed to Marx's philosophy. Bakunin predicted that Marx's theory of the transition from capitalism to socialism involving the working class seizing state power in a dictatorship of the proletariat would eventually lead to a usurpation of power by the state apparatus acting in its self-interest, ushering in a new form of capitalism rather than establishing socialism.

As a political ideology, state socialism rose to prominence during the 20th century Bolshevik, Leninist, and later Marxist–Leninist revolutions, where single-party control over the state and, by extension, over the political and economic spheres of society was justified as a means to safeguard the revolution against counter-revolutionary insurrection and foreign invasion. The Stalinist theory of socialism in one country was an attempt to legitimise state-directed activity to accelerate the industrialisation of the Soviet Union.

As a political ideology, state socialism is one of the major dividing lines in the broader socialist movement. It is often contrasted with non-state or anti-state forms of socialism, such as those that advocate direct self-management adhocracy and direct cooperative ownership and management of the means of production. Political philosophies contrasted with state socialism include libertarian socialist philosophies such as anarchism, De Leonism, economic democracy, free-market socialism, libertarian Marxism and syndicalism. These forms of socialism are opposed to hierarchical technocratic socialism, scientific management and state-directed economic planning.