The article considers the issue of the Soviet Union’s diplomatic support to the Republic of China at the initial stage of the Sino-Japanese War (1937–1939). It is noted that due to the outbreak of war, the Chinese government received real support neither from the League of Nations nor from the imperialist states since no one wanted to quarrel with Japan. It is emphasised that the leadership of the USSR had repeatedly unsuccessfully tried to negotiate with the imperialist powers on collective action against Japan, including, above all, the sanctions policy. Soviet diplomacy was ready to single-handedly support various foreign policy demarches of the leadership of the Republic of China. The data on numerous speeches by representatives of the USSR in the League of Nations in support of China are given. The active position of Soviet diplomacy during the so-called Brussels Conference is considered; their the Council of the League of Nations delegated the problem of Japanese aggression against China. It is emphasised that during the work of the Brussels conference, the delegations of Western countries demonstrated being uneager to aggravate relations with Japan; they only offered to search for peaceful ways to resolve the conflict, refused to accept collective sanctions against Japan or provide real international assistance to the victim of aggression – China. It is emphasised that Soviet diplomats continuously supported various initiatives of the government of the Chinese Republic in the League of Nations, only to be excluded from this organisation on December 14, 1939, after the USSR started war against Finland.