Jules Gilliéron (21 December 1854 – 26 April 1926) was a Swiss-French linguist and dialectologist. Born and initially educated in Switzerland, he studied linguistics at the École pratique des Hautes Études in Paris, where he spent the remainder of his life. His mentors included Michel Bréal and Gaston Paris, who supported his academic career; Bréal helped him secure a lectureship at the École pratique in 1883. He became a French citizen in 1886, a professor in 1894, and assistant director of the École pratique by 1913, the same year he was awarded the Légion d'honneur.

Gilliéron's early publications in the field of linguistic geography included an 1880 study of the dialect of the Swiss town of Vionnaz and the Petit Atlas phonétique de Valais roman ('Little Phonetic Atlas of Roman Valais'). In the latter, he developed the methods he would later apply to larger projects, such as the use of a pre-prepared questionnaire. He co-founded an academic journal, the Revue des patois gallo-romans ('Journal of Gallo-Romance Dialects'), with Jean-Pierre Rousselot in 1887. He used a novel phonetic transcription system, developed with Rousselot, for the Atlas Linguistique de la France ('Linguistic Atlas of France'; ALF), a complete linguistic atlas of the Gallo-Romance area. The ALF, which he produced in collaboration with a fieldworker, Edmond Edmont, was the first major linguistic atlas of a Romance language, and its methodology influenced subsequent works of linguistic geography. Karl Jaberg and Jakob Jud, two of Gilliéron's students, later applied his methods to Italy and southern Switzerland.

Gilliéron considered the proper object of etymology to be the whole history of words, rather than only their origins. He opposed the Neogrammarian view that language evolves through exceptionless laws of sound change, arguing that linguistic changes are not always uniform and are influenced by human psychology. He believed that speakers could only differentiate concepts for which they had discrete linguistic forms, and therefore viewed phenomena such as folk etymology as potentially "pathological" forces which could reduce the complexity and thus the usefulness of a language, and which require speakers to use "therapies" to counteract them.

Gilliéron has been called "the master of linguistic geography", and credited as the founder of scientific dialectology in France. His student body was highly international, and many of those who attended his lectures went on to be prominent in linguistics and related fields in countries throughout Europe.

Jules Louis Gilliéron was born on 21 December 1854 in La Neuveville, near Bern in Switzerland, the youngest of the four sons of Jean-Victor Gilliéron and his wife Méry (née Ganty). Jean-Victor had left a post as a primary-school teacher in the villages of Lutry and Aubonne in 1853, becoming a French-language teacher in the Progymnasium in La Neuveville. He taught in the Gymnasium for girls in Basel from 1866, and was also a respected amateur geologist and palaeontologist. Jules's elder brother Émile, born in 1850, was an artist and archaeological draughtsman; another elder brother died in infancy. As children, the Gilliéron boys, including Jules, took part in Jean-Victor's geological expeditions.

Jules attended school in Neuchâtel, where his teachers included the dialectologist Cyprien Ayer [fr]. In 1874–1875, he accompanied his eldest brother, Alfred, on a tour of the Eastern Mediterranean region, which spanned several months. He took his school leaving exams in the autumn of 1875: these were graded "satisfactory", in response to which Gilliéron wrote to the school's governing body, with his father's support, to refuse his diploma, on the grounds that it had been given to him "out of kindness" and judged inferior to those awarded before. In the winter of the same year, he enrolled at the University of Basel, where he attended lectures by the historian Jacob Burckhardt, the linguist Jules Cornu [de], and the classical philologist and philosopher Friedrich Nietzsche.

Cornu advised Gilliéron to study under the linguists Paul Meyer, based in the French city of Chartres, and Gaston Paris, a lecturer at École pratique des Hautes Études in Paris who had developed Meyer's theories that dialects were not monolithic and that linguistic geography should therefore concentrate on individual linguistic features rather than dialect groups. In 1876, at the age of 22, Gilliéron moved permanently to Paris, where he attended from November lectures at the École pratique by Gaston Paris; by Michel Bréal, a prominent philologist of Ancient Greek; by the linguist Arsène Darmesteter; and by the classical philologist Louis Havet. He also attended lectures by Meyer in Chartres. During his first academic year in Paris, he delivered a lecture at a conference organised by Havet on the pronunciation of Vulgar Latin. After the 1876–1877 academic year, he studied primarily under Gaston Paris. In 1878, he began teaching German at the Lycée Chaptal, a secondary school. There, he taught advanced classes for students aspiring to study at the prestigious École polytechnique or at the Saint-Cyr military academy, using a version of the direct method he developed himself. During his time at the Lycée Chaptal, he also worked as a substitute professor at the University of Paris. His salary rose from 800 francs in 1878 to 4,950 in 1891. In 1897, he resigned his teaching post to focus his work on the Atlas Linguistique de la France ('Linguistic Atlas of France'; ALF).

In January 1883, Gilliéron was appointed to the École pratique, succeeding Darmesteter as a lecturer in linguistics under Gaston Paris. At the suggestion of Bréal, who had supported his elevation to the post, he lectured on dialectology. His lectures initially covered Old French and the study of the living dialects of France. He became a French citizen in 1886, though continued to describe himself as Swiss, and to take holidays at a small property in Switzerland. He was made an Officier d'Academie, a member of the honorific Ordre des Palmes académiques, in 1891. From the 1892–1893 academic year, a new section of the École pratique for the study of the dialectology of Roman Gaul was created, separate from that (directed by Gaston Paris) dedicated to Romance linguistics: Gilliéron was placed in charge of it, with the title of deputy director. In 1894, he took the post of "Professor of Dialectology of Roman Gaul", at the École pratique, created for him by Gaston Paris. By 1913 he was assistant director of the École pratique, though his former student Iorgu Iordan later wrote that his salary at this point left him with barely enough to live on. In the same year, he was awarded the Légion d'honneur. By 1917, he was promoted to the rank of director of studies, the same title as Gaston Paris had held in the 1890s.