In the field of international relations, a sphere of influence (SOI) is a spatial region or concept division over which a state or organization has a level of cultural, economic, military, or political exclusivity.

While there may be a formal alliance or other treaty obligations between the influenced and influencer, such formal arrangements are not necessary and the influence can often be more of an example of soft power. Similarly, a formal alliance does not necessarily mean that one country lies within another's sphere of influence. High levels of exclusivity have historically been associated with higher levels of conflict.

In more extreme cases, a country within the "sphere of influence" of another may become a subsidiary of that state and serve in effect as a satellite state or de facto colony. This was the case with the Soviet Union and its Eastern Bloc after World War II. The system of spheres of influence by which powerful nations intervene in the affairs of others continues to the present. It is often analyzed in terms of superpowers, great powers, and/or middle powers.

Sometimes portions of a single country can fall into two distinct spheres of influence. In the 19th century, the buffer states of Iran and Thailand, lying between the empires of Britain, France and Russia, were divided between the spheres of influence of those three international powers. Likewise, after World War II, Germany was divided into four occupation zones, three of which later consolidated into West Germany and the remaining one became East Germany, the former a member of NATO and the latter a member of the Warsaw Pact.

Before modern international relations were established in Europe, many powerful states had subordinate tributary states, in which the less powerful states or kingdoms submit to the payment of tribute in order to maintain a degree of independence.

As European nation-states built empires in Africa, Asia, and the Pacific, often under the aegis of international law, the concept of "protectorate" and "spheres of influence" for characterizing relations of military and political domination emerged. Many areas of the world are joined by a cultural influence inherited from a previous sphere of influence, even if they are no longer under political control. Examples include Anglosphere, Arab World, Batavosphere, Persosphere, Eurosphere, Francophonie, Françafrique, Germanosphere, Indosphere, Hispanidad, Latin Europe/Latin America, Lusophonie, Turkosphere, Sinosphere, Slavisphere, Malay world, Post-Soviet States and many others.

Alexander Hamilton, first U.S. Secretary of the Treasury, aimed for the United States to establish a sphere of influence in North America. Hamilton, writing in the Federalist Papers, harboured ambitions for the US to rise to world power status and gain the strength to expel European powers from the Americas, taking on the mantle of regional dominance among American nations, although most of the New World were European colonies during that period.

This doctrine, dubbed the 'Monroe Doctrine', was formalized under President James Monroe, who asserted that the New World was to be established as a Sphere of influence, removed from European encroachment. As the U.S. emerged as a world power, few nations dared to trespass on this sphere (a notable exception occurred with the Soviet Union and the Cuban Missile Crisis).