The article analyzes the evolution and current state of the legal regulation of migration relations between the Russian Federation and the Republic of Belarus within the framework of the Union State. The authors highlight the historical stages of the formation of a unique regime of freedom of movement for citizens of the two countries, based on the 1999 Treaty on the Union State and the 2006 Agreement. The article identifies the key theoretical and legal challenge of establishing a harmonious balance between the openness of internal borders and the preservation of national sovereignty in visa policies towards third countries. The unresolved nature of this issue inevitably creates various negative consequences, including the risk of illegal transit by migrants. The test pays special attention to the new interstate regulatory dynamics that emerged in 2025, including the adoption of the Framework Program for a Coordinated Migration Policy for 2025–2030 and the initial signing of the Agreement on Mutual Recognition of Decisions.