Esther Diane Brimmer (born 1961) is an American foreign policy expert and former Assistant Secretary of State for International Organization Affairs.

Brimmer has served in the United States government and in international affairs think tanks.

From 1999-2001 she was a Member of the Office of Policy Planning at the U.S. Department of State working on the European Union, Western Europe, the United Nations, and multilateral security issues.

From 1995-1999 she managed projects as a Senior Associate at the Carnegie Commission on Preventing Deadly Conflict.

Previously, Dr. Brimmer served as Deputy Director and Director of Research at the Center for Transatlantic Relations at the Paul H. Nitze School of Advanced International Studies (SAIS) at Johns Hopkins University, Washington, D.C.[7] She is a member of the Atlantic Council Board of Directors,[8] Council on Foreign Relations, and Women in International Security.