The International Court of Justice in the Hague said Thailand must withdraw any soldiers from the area.
The long-standing rift has previously led to clashes between the two nations, who both lay claim to the land.
A 1962 verdict by the court declared the temple to be Cambodian, but did not rule on the area around it.
Cambodia sought a clarification of the ruling two years ago, after fighting erupted.
Delivering the judgement, Peter Tomka, president of the International Court of Justice, said the court had decided "that Cambodia had sovereignty over the whole territory of the promontory of Preah Vihear".
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