This article is an analytical review of five letters by King Henry III of France and Poland to the Russian tsars Ivan IV and Fyodor I between 1574 and 1588. The author reconstructs their content based on extant sources from archive collections in Russian and French libraries. The appendix provides evidence about the attribution of one of Henry III's letters from 1588, which was previously considered to have been addressed to Fyodor by Henry IV de Bourbon in 1589: its transcription based on three copies with a Russian translation. Based on the materials of royal and diplomatic correspondence, the article also traces the evolution of relations between the monarchs of the two countries in the second half of the sixteenth century.