The aim of the article is to define special features of the rivalry between Russia and theU. S. at the Vietnam's arms market. Nowadays Vietnam pays significant attention to its military modernization (mostly - under the influence of the situation in the South China Sea). Russia is a key partner of Vietnam in the sphere of defense cooperation. At the same time Vietnam tries to diversify its policy of arms import and develops interaction in this field with other countries, including the United States. The share of the U. S. at the Vietnam's arms market is still relatively low, but Washington is actively developing military cooperation with Hanoi. The U. S. also tries to strengthen its positions in Vietnam in other fields. Herewith theU. S. is demonstrating its support for Vietnam in the South China Sea conflict. Besides, the United States under the conditions of tensions with Russia is interested in expelling of Moscow from the Vietnam's arms market because of political reasons. So the U. S. policy might represent a serious potential challenge for defense interaction of Russia with Vietnam.