Thapsus (less commonly, Tapsus) was an ancient city in what is modern day Tunisia. Its ruins still exist at Ras Dimas near Bekalta, approx. 200 km SE of Carthage. Originally founded by Phoenicians, it served as a marketplace on the coast of the province Byzacena in Africa Propria. Thapsus was established near a salt lake on a point of land eighty stadia (14.8 km) from the Island of Lampadusa.

In 46 BC, Julius Caesar defeated Metellus Pius Scipio and the Numidian King Juba with a tremendous loss of men near Thapsus (see Battle of Thapsus). Caesar exacted a payment of 50,000 sesterces from the vanquished. Their defeat marked the end of opposition to Caesar in Africa. Thapsus then became a Roman colony.

The city should not be confused with the medicinal herb Verbascum thapsus.

About the same time Lamis arrived in Sicily with a colony from Megara, and after founding a place called Trotilus beyond the river Pantacyas, and afterwards leaving it and for a short while joining the Chalcidians at Leontini, was driven out by them and founded Thapsus. After his death his companions were driven out of Thapsus, and founded a place called the Hyblaean Megara; Thucydides Book 6