The Convention on the Protection of the Marine Environment of the Baltic Sea Area signed in 1974 in Helsinki is probably the most important environmental agreement consummated in the Baltic Sea region. This article is the first study that explores the history of this agreement, also known as the Helsinki Convention, by using primary archival sources. The principal sources are the archives of the Ministry for Foreign Affairs of Finland. We examine the role of Finland in the process that led to the signing of the Helsinki Convention from the perspective of international politics. The study focuses primarily on Finnish, Swedish, and Soviet state-level parties from the end of the 1960s to 1974. We show that Cold War politics affected in several ways negotiations and contents of the Helsinki Convention. We also argue that the Soviet Union used the emerging international environmental issues as a new tool of power politics.