United States defence secretary Leon Panetta, who is visiting Japan, says the two countries have agreed to establish a second missile defence system on Japanese soil.
He made the announcement in Tokyo with Japan's defence minister Satoshi Morimoto.
Mr Panetta says the system will enhance their ability to defend both Japan and the US from the North Korean ballistic missile threat.
The BBC reports the two allies are also watching China's rapid military growth with increasing concern.
Despite the insistence of both Washington and Tokyo that the system is not aimed at China, analysts believe the decision is bound to anger Beijing.
The exact location of the system has not yet been determined.
The BBC says Mr Panetta's trip comes amid fresh tensions between Japan and China over disputed islands known as Diaoyu in China and Senkaku in Japan, after Japan sealed a deal to buy three of the islands.