The article covers the important aspects of the Russian-Swedish war of 1741-1743, which has attracted little attention in Russian historiography. The war was waged over three campaigns, witnessed by two British ambassadors in Russia, Edward Finch and Cyril Wyche. Based on the diplomatic correspondence of the British ambassadors, previously unexplored in the Russian historical science, the author analyzes the information which they received from high-ranking Russian officials, including commanders of the troops, and then passed on to London. In particular, it concerned the state of the Russian armed forces on the eve of the war with Sweden. The diplomats noted and informed their superiors about the weaknesses of the Russian navy. Keeping track of the course of the war, the ambassadors briefed the Secretary of State about its causes, the main being the desire of the Swedes to abrogate the conditions of the Nishtad peace and regain the territories lost during the Northern War of 1700-1721. Both ambassadors acknowledged the courage and heroism shown by the Russian soldiers in battles with the enemy, as well as their humane treatment of the captured Swedes. At the same time, they drew attention to the manifestations of cruelty inflicted by irregular troops who participated in the raids in Finland. The dispatches of the ambassadors also reveal the stance taken by a number of countries towards the conflict between Russia and Sweden: hostile on the part of France and generally benevolent on the part of official London, which did not want to spoil relations with the Russian court on the eve of the conclusion of the 1742 Treaty of Alliance. Other documents of indisputable interest are manifestos (included in the correspondence of diplomats) addressed by the Swedish king and the Russian empress to the population of the border territories and aimed at showing goodwill towards the enemy troops. The facts about the Russian-Swedish war presented by the British ambassadors and analyzed in the article fill the existing gap in one of the episodes of Russian-Swedish relations in the middle of the eighteenth century.