Who has the authority to fire the president of the US?

The president cannot be "fired" per se, but he/she can be removed from office should he/she be found guilty of "treason, bribery, or other high crimes and misdemeanors" as stated in the U.S. Constitution. The first step of the process requires the U.S. House of Representatives to pass the articles of impeachment against the president, which requires a simple majority. Upon passage, the U.S. Senate holds a trial, which is presided by the Chief Justice of the United States. In order to convict the accused, a two-thirds majority of the senators present is required.

A president who is defeated for reelection could also be considered "fired" in an informal sense, as his/her loss could be regarded as a referendum on his/her job performance.