Josep Almudéver Mateu, the last-known survivor of the 35,000 foreign volunteers who joined the International Brigades to fight against fascism during the Spanish Civil War of 1936-1939, died May 23 in a hospital near his home in Pamiers, France. He was 101.

The Madrid-based Association of Friends of the International Brigades, which described Mr. Almudéver as “the last man standing,” announced the death but did not specify a cause.

Mr. Almudéver was not quite 17 when he lied about his age to enlist in the regular army of the elected Spanish republican government, which was in bloody conflict with the insurrectionist “nationalist” (essentially fascist) forces of Gen. Francisco Franco.

He was initially assigned to a battalion defending Alcàsser (his hometown) and Valencia, cities near Spain’s southeastern coast. In late 1936, he was sent to the front lines to attack a group of Franco’s forces that had taken Teruel, a key town on the road to the capital, Madrid. He was in the trenches in February 1937 when officers realized his age and sent him packing home.

But having been born in France (where his Spanish parents were briefly working), he soon transferred to the International Brigades, many of whose fighters saw the battle for Spain as the spearhead of an effort to crush fascism throughout Europe. Despite official U.S. neutrality in the conflict, almost 3,000 Americans joined the International Brigades.

In May 1938, by then 18 and using his French passport, Mr. Almudéver enlisted in the International Brigades, which tried to make up in idealistic fervor what they often lacked in supplies.

“We went to the front without bullets,” Mr. Almudéver recalled to the Spanish newspaper El Pais in 2011. “After five kilometers, a column [of fighters] from the Spanish Communist Party gave me five, then a colonel gave me five more. Ten bullets to fight a war!”

Within weeks, he was wounded in the arm and leg, and as Franco’s forces pushed forward, he ended up going underground in the Mediterranean town of Alicante. By the time Franco declared victory in April 1939, the International Brigades had been disbanded and most of the foreign fighters had returned home. But others, including Mr. Almudéver, were rounded up and sent to concentration camps.

He was first taken to the Albatera camp near Alicante, where, he said, many were put before a firing squad or died of starvation. “I don’t know why, but they always made me watch when they shot people who had tried to escape from the camp,” Mr. Almudéver told El Pais. “Never, in all my life, will I forget the screams of the people who were shot.”

He also spent three years in other camps or prisons before being released in 1942. With Franco in power, and still facing reprisals, he fled into exile in Pamiers, close to the Pyrenees mountain region near the Spanish border. He worked in the construction sector while campaigning for the rest of his life against fascism.

The Association of Friends of the International Brigades said Mr. Almudéver, who eventually gave speeches about his role in the civil war, was glad he lived to see not just Spain transition to democracy after Franco’s death in 1975 but also the dictator’s body exhumed in 2019 from his elaborate mausoleum to be reburied in a municipal cemetery.

Josep Eduard Almudéver Mateu was born in the port city of Marseille, France, on July 30, 1919. His father was a builder from Spain’s Valencia region who had moved to France to seek work at the start of World War I, and his mother was from a Spanish family circus that was touring France at the time.

The family returned to Valencia when Josep was a boy, and his friends called him José. He was at school in Alcàsser, just south of the city of Valencia, when Franco and other generals, backed by Germany’s Adolf Hitler and the Italian fascist dictator Benito Mussolini, launched a coup against the republican government in July 1936, triggering the civil war.

The last American survivor of the International Brigades, Californian Delmer Berg, died in 2016 at 100. Varying estimates say between 700 and 900 Americans were among some 7,000 members of the International Brigades who were killed in the conflict. Those who returned home faced discrimination in the post-World War II years of anti-communist hysteria.

Mr. Almudéver is survived by his brother, Vicente, 104, who fought in the regular republican army during the key battles of Jarama and Madrid, as well as five children and several grandchildren and great-grandchildren. His wife, Carmen Ballester Vicens, died in 2006.