Thomas Alfred Shannon Jr. (born 1958) is an American diplomat who served as Under Secretary of State for Political Affairs from 2016 to 2018.

[3] After retirement, Shannon joined law firm Arnold & Porter as Senior International Policy Advisor.

[7] He served as the Regional Labor Attaché at the U.S. Consulate-General in Johannesburg, South Africa, from 1992 to 1996; as Political Counselor at the U.S. Embassy in Caracas, Venezuela, from 1996 to 1999; and as the Director for Inter-American Affairs at the National Security Council (NSC) from 1999 to 2000.

[7] Shannon was appointed as U.S. Deputy Permanent Representative to the Organization of American States (OAS) from 2000 to 2001, serving under Luis J. Lauredo.

[7] Shannon then served as Special Assistant to the President and Senior Director for Western Hemisphere Affairs at the NSC from 2003 to 2005.

In October 2005, he became Assistant Secretary of State for Western Hemisphere Affairs and served in that role until November 2009, when he was replaced by Arturo Valenzuela.

"[10] Shannon retained his position as Counselor until February 12, 2016, when he was appointed as Under Secretary of State for Political Affairs for the last year of Barack Obama's presidency.

He remained in office for only 12 days, when on February 1, 2017, the Senate confirmed Rex Tillerson, President Trump's nominee, as Secretary of State.

[8][14] While Shannon was acting Secretary of State, President Trump dismissed many prominent and senior State Department officials, resulting in the firing or forced resignations of over a dozen career diplomats,[15] leaving a majority of senior career positions at the Department vacant.

On June 29, 2016, he met with India Foreign Secretary Jaishankar, and expressed support for the Indian government, calling it America's "anchor" in the Asia-Pacific region.