The Reinsurance Treaty was a diplomatic agreement between the German Empire and the Russian Empire that was in effect from 1887 to 1890. Only a handful of top officials in Berlin and St. Petersburg knew of its existence since it was top secret. The treaty played a critical role in German Chancellor Otto von Bismarck's extremely complex and ingenious network of alliances and agreements, which aimed to keep the peace in Europe and to maintain Germany's economic, diplomatic and political dominance. It helped keep the peace for both Russia and Germany.

The treaty provided that both parties would remain neutral if the other became involved in a war with a third great power, but that would not apply if Germany attacked France or if Russia attacked Austria-Hungary. Germany paid for Russian friendship by agreeing to the Russian sphere of influence in Bulgaria and Eastern Rumelia (now part of southern Bulgaria) and by agreeing to support Russian action to keep the Black Sea as its own preserve. After Bismarck lost power in 1890, his enemies in the Foreign Ministry convinced the Kaiser that the treaty was too much in Russia's favor and should not be renewed. The cancellation like the treaty itself remained a top secret. Russia however wanted the protections it afforded, and was angry at the termination. Needing allies, Russia opened negotiations with Germany's enemy France. The resulting Franco-Russian Alliance of 1891–1892 to 1917 rapidly began to take shape. Historians consider it a major disaster for Germany, and one of the long-term causes of the First World War.