(anc. Tibiscus), a river of Hungary, flowing into the Danube, formed by the union about 20 miles E.N.E. of Szegith of the Black and the White Theiss, which both rise in the Carpathians, on the borders of Galicia. It flows in an irregular course, at first westwards, receiving numerous affluents from either side, the most important of which are the Szamos from the S. and the Bodrog from the N. The latter joins it near Tokay, where it changes its direction, and flows for some distance generally southwest, with many windings. Farther down it receives the Hernad from the right, and below Szolnok, where its direction becomes almost due S., the Kiros and the Maros from the left. It joins the Danube 22 miles E. of Peterwardein, after a total course of 500 miles, for the greater part of which it is navigable. The chief traffic on the river consists in the conveyance down its stream of salt, timber, and agricultural produce. The Theiss abounds in sturgeon and other fish, and has valuable fisheries.