The article examines the development of Azerbaijani-Persian relations in 1918-1920, when a newly formed state called Azerbaijan Democratic Republic appeared on the world map. Before the declaration of independence, the territory of Azerbaijan was a part of the Russian Empire, which received Azerbaijani lands as a result of the Russian-Persian wars in the first half of the 19th century. The Turkmenchay Treaty of 1828 defined the border separating Azerbaijan and Armenia from Persia and consolidated the division of Azerbaijan between the Russian Empire and Persia. As a result, the territory of Azerbaijan was divided into two parts - northern and southern, called Caucasian and Persian Azerbaijan respectively. The article is an attempt to fill the gap in the analysis of Azerbaijani-Persian relations in 1918-1920 in the domestic science. Historical, narrative and system methods were used as the main methods in the work on the article. The relevance of the article is conditioned by the possibility of revision of international treaties of historical significance. In particular, modern Iran has started revising the Turkmenchay Treaty of 1828. The revision of previously of earlier treaties may pose a threat to regional international relations and alter the territorial integrity of the states involved in the process. Understanding the historical development of the countries is important for building a foreign policy line at the present stage, including for Russia. Based on an analysis of foreign policy documents of Azerbaijan Democratic Republic, the author comes to the conclusion that Azerbaijani-Persian relations were ambiguous: if initially Persia declared its territorial claims to Azerbaijani lands, then Persia’s policy soon turned in the opposite direction, and the countries signed a number of agreements on foreign policy cooperation. The most important result of bilateral cooperation was the de jure recognition of Azerbaijan by Persia.