American imperialism is the expansion of American political, economic, cultural, media, and military influence beyond the boundaries of the United States of America. Depending on the commentator, it may include imperialism through outright military conquest; gunboat diplomacy; unequal treaties; subsidization of preferred factions; regime change; or economic penetration through private companies, potentially followed by diplomatic or forceful intervention when those interests are threatened.

In historical contexts, New Imperialism characterizes a period of colonial expansion by European powers, the United States, and Japan during the late 19th and early 20th centuries. The period featured an unprecedented pursuit of overseas territorial acquisitions. At the time, states focused on building their empires with new technological advances and developments, expanding their territory through conquest, and exploiting the resources of the subjugated countries. During the era of New Imperialism, the European powers individually conquered almost all of Africa and parts of Asia. The new wave of imperialism reflected ongoing rivalries among the great powers, the economic desire for new resources and markets, and a "civilizing mission" ethos. Many of the colonies established during this era gained independence during the era of decolonization that followed World War II.