A vassal state is any state that has a mutual obligation to a superior state or empire, in a status similar to that of a vassal in the feudal system in medieval Europe. Vassal states were common among the empires of the Near East, dating back to the era of the Egyptian, Hittite and Mitanni conflict, as well as ancient China. The use of vassal states continued through the Middle Ages, with the last empire to use such states being the Ottoman Empire.

The relationships between vassal rulers and empires was dependent on the policies and agreements of each empire. While payment of tribute and military service is common amongst vassal states, the degree of independence and benefits given to vassal states varied. Today, more common terms are puppet state, protectorate, client state, associated state or satellite state.