'I am running for the U.S. Senate from the Great State of Arizona, for one unwavering reason: to support the agenda and policies of President Donald Trump in his mission to Make America Great Again,' Arpaio wrote on Twitter.
Arizona Republican Sen. Jeff Flake's decision not to seek reelection this year sets up a primary fight between Arpaio and two other contenders.
The 85-year-old Arpaio, an early Trump supporter, had been convicted of criminal contempt after defying a judge's order to stop racially profiling Latinos.
In August, Trump issued the first clemency of his presidency, pardoning the controversial former sheriff.
FILE - Donald Trump is joined by Joe Arpaio at a campaign event in Marshalltown, Iowa, Jan. 26, 2016.
In Arpaio's defense, a White House statement cited his age and 'life's work of protecting the public from the scourges of crime and illegal immigration.'
Critics slammed the move, saying Trump acted to protect a political crony and didn't follow Department of Justice guidelines in issuing the pardon.