lowest and first of three intercontinental stages of the Mississippian Subsystem (Mississippian Subperiod), Carboniferous System (Carboniferous Period), encompassing all rocks deposited during the Tournaisian Age (359.2 to 345.3 million years ago). The name is derived from exposures of fine-grained limestone with shaly intervals surrounding the town of Tournai in southwestern Belgium, near the French border.

By international agreement, the coterminous bases of the Carboniferous, the Mississippian, and the Tournaisian stages are drawn at the first appearance of the conodont (primitive chordate with tooth-shaped fossil remains) Siphonodella sulcata in sequence with its evolutionary precursor S. praesulcata. This horizon is also about the level of the appearance of the ammonoid Gattendorfia. Ratified by the International Commission on Stratigraphy (ICS) in 1990, the Global Stratotype Section and Point for that horizon has been selected at the base of bed 89 in the La Serre section, Montagne Noire, of southern France. This horizon can also be recognized by appearances of ammonoids (ammonoid), smaller calcareous foraminifers (foraminiferan) (pseudopod-using unicellular organisms protected by a test or shell), and palynomorphs (microfossils composed of microscopic remains of plant and animal structures).

The top of the Tournaisian is characterized by the base of the Viséan Stage, currently indicated by the first appearance of the foraminifer Eoparastaffella simplex, which occurs very close to the first appearance of the conodont Gnathodus homopunctatus in the fossil record. The Tournaisian overlays the Famennian Stage of the Devonian Period.