Théodore Delachaux was a Swiss artist, teacher, curator, and ethnologist.

Delachaux was the son of a doctor who introduced him to the natural sciences at an early age. His maternal grandfather was the entomologist and botanist Charles Henri Godet (1797-1879).

At the age of eleven, Delachaux wrote a book on plankton, illustrating it with drawings based on his microscopic observations. In Neuchâtel, Delachaux attended school and lived with his uncle Paul Godet (1836-1911), director of the Neuchâtel Museum of Natural History. It was with him that Delachaux continued his scientific education.

After passing his baccalaureate, Delachaux studied at the École nationale supérieure des beaux-arts de Paris and was taught by Luc-Olivier Merson. Delachaux returned to Neuchâtel in 1912, where he taught art at the gymnasium and, from 1916, at the vocational school for girls until 1944.

In the early 20th century, Delachaux spent some time with his brother, the doctor Constant Delachaux (1875-1952), in the district of Pays d'Enhaut and discovered the work of Johann Jakob Hauswirth. The brothers bought works by Hauswirth, mostly from private owners, to build their own collections.

In addition to his teaching, Delachaux painted and helped found an art school and an exhibition gallery for painting and applied arts. Throughout his life, he collected Swiss folk art, including toys and ceramics.

In 1919, Delachaux discovered the species Troglochaetus beranecki in the Grotte du Ver, a cave in the Areuse Gorge. When Charles Knapp (1855–1921), curator at the Musée d'ethnographie de Neuchâtel, died, Delachaux, supported by Gustave Jéquier, was able to succeed him. Delachaux subsequently had to label and catalog over 20,000 objects, expand the collections and create new rooms for them. He also worked to make the museum accessible to the general public, thus promoting education.

Delachaux led an ethnographic expedition to Angola from 1932 to 1933 and brought back extensive documentation about the people he encountered. As a curator, Delachaux regularly worked on various archaeological excavations. He often helped Paul Vouga in his work, either as a naturalist or as an illustrator. After Vouga's death, he was appointed curator of the prehistoric collections at the Museum of History and Archaeology in 1940 and took over teaching at the University of Neuchâtel, a position he held until 1949. Delachaux also worked for several years as an assistant in Professor Otto Fuhrmann's geology laboratory. Among other things, he illustrated Fuhrmann's scientific work on tapeworms.

When Delachaux handed over the management of the Ethnographic Museum to Jean Gabus (1908–1992) in 1945, he took over the management of the Natural History Museum in Neuchâtel from Otto Fuhrmann and worked on its further development until a year before his death.