Jean Charles Léonard de Sismondi, also known as Jean Charles Leonard Simonde de Sismondi (French: [ʒɑ̃ ʃaʁl leɔnaʁ də sismɔ̃di]; 9 May 1773 – 25 June 1842), whose real surname was Simonde, was a Swiss historian and political economist, who is best known for his works on French and Italian history, and his economic ideas. His Nouveaux principes d'économie politique, ou de la richesse dans ses rapports avec la population (1819) represents the first liberal critique of laissez-faire economics. He was one of the pioneering advocates of unemployment insurance, sickness benefits, a progressive tax, regulation of working hours, and a pension scheme. He was also the first to coin the term proletariat to refer to the working class created under capitalism, and his discussion of mieux value anticipates the concept of surplus value. According to Gareth Stedman Jones, "much of what Sismondi wrote became part of the standard repertoire of socialist criticism of modern industry," earning him critical commentary in the Communist Manifesto.

His paternal family seem to have borne the name Simonde, at least from the time when they migrated from Dauphiné to Geneva at the revocation of the edict of Nantes. It was not till after Sismondi had become an author that, observing the identity of his family arms with those of the once flourishing Pisan house of the Sismondi and finding that some members of that house had migrated to France, he assumed the connection without further proof and called himself Sismondi.

The Simondes, however, were themselves citizens of Geneva of the upper class, and possessed both rank and property, though the father was also a village pastor. His uncle by marriage was the prominent pastor Jacob Vernes, a friend of Voltaire and Rousseau.

The future historian was well educated, but his family wished him to devote himself to commerce rather than literature, and he became a banker's clerk in Lyon. Then the Revolution broke out, and as it affected Geneva, the Simonde family took refuge in England where they stayed for eighteen months (1793–1794). Disliking—it is said—the climate, they returned to Geneva, but found the state of affairs still unfavourable; there is even a legend that the head of the family was reduced to selling milk himself in the town. The greater part of the family property was sold, and with the proceeds they emigrated to Italy, bought a small farm in Pescia near Lucca and Pistoia, and set to work to cultivate it themselves.

Sismondi worked hard there, with both his hands and mind, and his experiences gave him the material of his first book, Tableau de l'agriculture toscane, which, after returning to Geneva, he published there in 1801. At a young age, Sismondi had read The Wealth of Nations and became strongly attached to Smith's theories. He apparently published his first work on the subject of political economy, De la richesse commerciale ou principes de l'economie politique appliqué à la legislation du commerce (1803) to explain and popularize Smith's doctrine, but following this Sismondi spent a considerable amount of time dedicated to historical research. He again turned his attention to political economy around 1818 when he was commissioned to write an entry on "Political Economy" for the Edinburgh Encyclopædia. This was just following a serious economic downturn after the outbreak of the first major crisis in 1815.

As an economist, Sismondi represented a humanitarian protest against the dominant orthodoxy of his time. In his 1803 book, he followed Adam Smith; but in his principal subsequent economic work, Nouveaux principes d'économie politique (1819), he insisted on the fact that economic science studied the means of increasing wealth too much, and the use of wealth for producing happiness, too little. For the science of economics, his most important contribution was probably his discovery of economic cycles. In contrast to other thinkers at the time (notably J. B. Say and David Ricardo), Sismondi challenged the idea that economic equilibrium leading to full employment would be spontaneously achieved. He wrote, "Let us beware of this dangerous theory of equilibrium which is supposed to be automatically established. A certain kind of equilibrium, it is true, is reestablished in the long run, but it is after a frightful amount of suffering." Sismondi also argued that classical economics failed to consider the negative social and environmental impacts of economic growth, and that it failed to address issues of income inequality and social justice. Although his thinking is characterized as "petty-bourgeois Socialism" in the Communist Manifesto, Sismondi was not a socialist. Nonetheless, in protesting against laissez faire and invoking the state "to regulate the progress of wealth," he was an interesting precursor of the German Historical school of economics.

Focuses of his work are central to the idea of taking particular economic situations and analyzing them in the situational setting of history from which one is drawing data or insight.

Sismondi is known for the study of economic crises rooting from “the social ramifications of the economic system rather than on its structure.” His interpretations put him before Marx, in semi-defining the bourgeoisie and proletariat division in society. Key to his philosophy, Sismondi saw these class divisions to coincide with crises in the economy and didn't see extreme social reform as the answer, but rather moderate versions that allowed for technological advances to be slowed for the economy to catch up, via limiting production and the limitation of what he referred to as “the prevailing glorification of free competition”, all while, most importantly, allowing individuals to retain private property and any revenues generated from it.