However, the passion for archaeology, into which he had been initiated as a teenager by his maternal uncle Jacques-Gabriel Bulliot, an eminent figure in the Société éduenne des lettres, sciences et arts, quickly took over, although he continued working for the family business until 1899.In 1884, he joined La Diana, an archaeological and historical society based in Montbrison, whose aim was to identify and study the antiquities and monuments of the Forez region, south of Roanne.

From February to April 1893, Joseph Déchelette made a trip to Egypt from which he returned loaded with the mummy of Nesyamun, supposedly dead at the age of fifteen and who, during her lifetime, sang in Thebes for the god Amon.

Déchelette was the first to have demonstrated a cultural unity north of the Alps towards the end of the Iron Age by comparing the results of the archaeological excavations of four oppida: Bibracte at Mont Beuvray, Manching in Bavaria, Stradonice in Bohemia, and Szent Vid-hegy in Hungary.

In his 1904 work devoted to the ornate vases of Gaul, he made a compilation of the discoveries of ancient ceramics where the analysis attached both to the decoration and to the form, a typology which is still used today.

In 1914, at the outbreak of the First World War, recalled to the 104th Territorial Infantry Regiment, he requested, despite his advanced age, an assignment to the front to fill the gaps left by the First Battle of the Marne.

He rests today in the national Nécropole nationale du Bois-Roger in Ambleny and his name is inscribed in the Panthéon, among the 560 writers who died for France in the First World War.