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 in ancient China.

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 in ancient China. The relationships between vassal rulers and empires were dependent on the policies and agreements of each empire.

The stronger state typically agrees to provide military support and any other existential support to keep the vassal state afloat in times of crisis. In return, the dominant state receives commercial, military or tributary (taxes) privileges. Vassal states typically agree to the deal out of a combination of consent and/or implicit force.

The vassal has authority on the subordinate state in many aspects: military, political and economic. The relationship between the ruler state and the vassal state is mainly anchored on military affairs. The vassal state provides military assistance to the dominant state whenever requested to do so.