The natural prolongation principle or principle of natural prolongation is a legal concept introduced in maritime claims submitted to the United Nations.

The phrase denotes a concept of political geography and international law that a nation's maritime boundary should reflect the 'natural prolongation' of where its land territory reaches the coast.

Oceanographic descriptions of the land mass under coastal waters became conflated and confused with criteria that are deemed relevant in border delimitation. The concept was developed in the process of settling disputes if the borders of adjacent nations were located on a contiguous continental shelf.

An unresolved issue is whether a natural prolongation defined scientifically, without reference to equitable principles, is to be construed as a "natural prolongation" for the purpose of maritime border delimitation or maritime boundary disputes.