German foreign secretary during part of World War I (1916–17), the author of a sensational proposal to Mexico to enter into an alliance against the United States.

After a career in the consular service, Zimmermann won transfer to the diplomatic branch in 1901. Because of the retiring nature of Gottlieb von Jagow, who became foreign secretary in 1913, Zimmermann conducted a large share of the relations with foreign envoys. As acting secretary in Jagow's absence, he participated, with Emperor William (Wilhelm) II (William II) and Chancellor Theobald von Bethmann Hollweg (Bethmann Hollweg, Theobald von), in Germany's decision of July 5, 1914, to support Austria-Hungary when, after the assassination of Archduke Francis Ferdinand (Francis Ferdinand, archduke of Austria-Este) at Sarajevo, Austria-Hungary put pressure on Serbia, thus angering Russia. Zimmermann drafted the telegram to Vienna embodying Germany's decision, which intensified the crisis that culminated in the outbreak of war.

In 1916, when the German High Command insisted on the resumption of unrestricted submarine warfare as the only remaining weapon to defeat the Allies, even at the risk of provoking the United States into belligerency, Jagow resigned. On November 25, Zimmermann, who was regarded as “pro-U-boat,” was appointed to succeed him. In an effort to nullify or at least to reduce U.S. intervention in Europe by engaging U.S. arms and energies elsewhere, Zimmermann planned to embroil the United States in war with Mexico and Japan. In pursuit of this goal, on January 16, 1917, he sent a secret telegram in code (through the German ambassador in Washington, D.C.) to the German minister in Mexico, authorizing him to propose an alliance to Mexico's President Venustiano Carranza (Carranza, Venustiano). The offer included “an understanding on our part that Mexico is to reconquer her lost territory in Texas, New Mexico, and Arizona.” Carranza was also asked to “invite the immediate adherence of Japan.” Intercepted and decoded by British Admiralty intelligence, the telegram was made available to President Woodrow Wilson (Wilson, Woodrow), who caused it to be published on March 1, 1917. In convincing Americans of German hostility toward the United States, the “Zimmermann Note” became one of the factors leading to the U.S. declaration of war against Germany five weeks later.

Zimmermann lost office just after the fall of Bethmann Hollweg's government in the summer of 1917 and never held it again.