The Agreement of Friendship, Cooperation, and Mutual Assistance of 1948, also known as the YYA Treaty from the Finnish Ystävyys-, yhteistyö- ja avunantosopimus (YYA-sopimus) (Swedish: Vänskaps-, samarbets- och biståndsavtalet (VSB-avtalet)), was the basis for Finno–Soviet relations from 1948 to 1992.

The agreement also recognized Finland's desire to remain outside great-power conflicts, allowing the country to adopt a policy of neutrality in the Cold War.

Due to the uncertain status of Finno–Soviet relations in the years after the Continuation War, and the precise interpretation of the treaty's wording, Finland followed the Eastern Bloc countries' decision and did not participate in the Marshall Plan.

The treaty was an instrumental tool for the Soviet Union to gain political leverage in the internal affairs of Finland in post-war era, in confrontations such as the note crisis.

This ranged from Finnish organizations such as the Social Democratic Party accepting U.S. Central Intelligence Agency funding to sharing of seismic data on nuclear tests.