Marc Léopold Benjamin Bloch (/blɒk/ BLOCK; French: [maʁk leɔpɔld bɛ̃ʒamɛ̃ blɔk]; 6 July 1886 – 16 June 1944) was a French historian. He was a founding member of the Annales School of French social history. Bloch specialised in medieval history and published widely on medieval France over the course of his career. As an academic, he worked at the University of Strasbourg (1920 to 1936 and 1940 to 1941), the University of Paris (1936 to 1939), and the University of Montpellier (1941 to 1944).

Born in Lyon to an Alsatian Jewish family, Bloch was raised in Paris, where his father—the classical historian Gustave Bloch—worked at Sorbonne University. Bloch was educated at various Parisian lycées and the École Normale Supérieure, and from an early age was affected by the antisemitism of the Dreyfus affair. During the First World War, he served in the French Army and fought at the First Battle of the Marne and the Somme. After the war, he was awarded his doctorate in 1918 and became a lecturer at the University of Strasbourg. There, he formed an intellectual partnership with modern historian Lucien Febvre. Together they founded the Annales School and began publishing the journal Annales d'histoire économique et sociale in 1929. Bloch was a modernist in his historiographical approach, and repeatedly emphasised the importance of a multidisciplinary engagement towards history, particularly blending his research with that on geography, sociology and economics, which was his subject when he was offered a post at the University of Paris in 1936.

During the Second World War Bloch volunteered for service, and was a logistician during the Phoney War. Involved in the Battle of Dunkirk and spending a brief time in Britain, he unsuccessfully attempted to secure passage to the United States. Back in France, where his ability to work was curtailed by new antisemitic regulations, he applied for and received one of the few permits available allowing Jews to continue working in the French university system. He had to leave Paris, and complained that the Nazi German authorities looted his apartment and stole his books; he was also persuaded by Febvre to relinquish his position on the editorial board of Annales. Bloch worked in Montpellier until November 1942 when Germany invaded Vichy France. He then joined the non-Communist section of the French Resistance and went on to play a leading role in its unified regional structures in Lyon. In 1944, he was captured by the Gestapo in Lyon and murdered in a summary execution after the Allied invasion of Normandy. Several works—including influential studies like The Historian's Craft and Strange Defeat—were published posthumously.

His historical studies and his death as a member of the Resistance together made Bloch highly regarded by generations of post-war French historians; he came to be called "the greatest historian of all time". By the end of the 20th century, historians were making a more critical assessment of Bloch's abilities, influence, and legacy, arguing that there were flaws to his approach.

Marc Bloch was born in Lyon on 6 July 1886, one of two children to Gustave and Sarah Bloch, née Ebstein. Bloch's family were Alsatian Jews: secular, liberal and loyal to the French Republic. They "struck a balance", says the historian Carole Fink [de], between both "fierce Jacobin patriotism and the antinationalism of the left". His family had lived in Alsace for five generations under French rule. In 1871, France was forced to cede the region to Germany following its defeat in the Franco-Prussian War. The year after Bloch's birth, his father was appointed professor of Roman History at the Sorbonne, and the family moved to Paris—"the glittering capital of the Third Republic". Marc had a brother, Louis Constant Alexandre, seven years his senior. The two were close, although Bloch later described Louis as being occasionally somewhat intimidating. The Bloch family lived at 72, Rue d'Alésia, in the 14th arrondissement of Paris. Gustave began teaching Marc history while he was still a boy, with a secular, rather than Jewish, education intended to prepare him for a career in professional French society. Bloch's later close collaborator, Lucien Febvre, visited the Bloch family at home in 1902; although the reason for Febvre's visit is now unknown, he later wrote of Bloch that "from this fleeting meeting, I have kept the memory of a slender adolescent with eyes brilliant with intelligence and timid cheeks—a little lost then in the radiance of his older brother, future doctor of great prestige".

Bloch's biographer Katherine Stirling ascribed significance to the era in which Bloch was born: the middle of the French Third Republic, so "after those who had founded it and before the generation that would aggressively challenge it". When Bloch was nine-years-old, the Dreyfus affair broke out in France. As the first major display of political antisemitism in Europe, it was probably a formative event of Bloch's youth, along with, more generally, the atmosphere of fin de siècle Paris. Bloch was eleven when Émile Zola published J'Accuse…!, his indictment of the French establishment's antisemitism and corruption. Bloch was greatly affected by the Dreyfus affair, but even more affected was nineteenth-century France generally, and his father's employer, the École Normale Supérieure, saw existing divides in French society reinforced in every debate. Gustave Bloch was closely involved in the Dreyfusard movement and his son agreed with the cause.

Bloch was educated at the prestigious Lycée Louis-le-Grand for three years, where he was consistently head of his class and won prizes in French, history, Latin, and natural history. He passed his baccalauréat, in Letters and Philosophy, in July 1903, being graded trés bien (very good). The following year, he received a scholarship and undertook postgraduate study there for the École normale supérieure (ÉNS) (where his father had been appointed maître de conferences in 1887). His father had been nicknamed le Méga by his students at the ÉNS and the moniker Microméga was bestowed upon Bloch. Here he was taught history by Christian Pfister and Charles Seignobos, who led a relatively new school of historical thought which saw history as broad themes punctuated by tumultuous events. Another important influence on Bloch from this period was his father's contemporary, the sociologist Émile Durkheim, who pre-figured Bloch's own later emphasis on cross-disciplinary research. The same year, Bloch visited England; he later recalled being struck more by the number of homeless people on the Victoria Embankment than the new Entente Cordiale relationship between the two countries.

The Dreyfus affair had soured Bloch's views of the French Army, and he considered it laden with "snobbery, anti-semitism and anti-republicanism". National service had been made compulsory for all French adult males in 1905, with an enlistment term of two years. Bloch joined the 46th Infantry Regiment based at Pithiviers from 1905 to 1906.