Joe Arpaio, 85, was found guilty after he defied a court order to stop traffic patrols targeting suspected immigrants. He was due to be sentenced in October.
President Trump had hinted at the pardon at a rally in Phoenix on August 22.
Thanking the president, Joe Arpaio said his conviction was “a political witch hunt by holdovers in the Obama justice department”.
Joe Arpaio tweeted: “Thank you…. for seeing my conviction for what it is.”
“I’m not going away,” he said, while declining to say whether he would run for sheriff again.
President Trump has frequently praised the former sheriff, who is known for his controversial hard-line stance on immigration.
Joe Arpaio appeared on the campaign trail with President Trump in 2016 – describing him as “the law-and-order candidate”.
In a statement announcing the pardon, his first, President Trump said: “Arpaio’s life and career, which began at the age of 18 when he enlisted in the military after the outbreak of the Korean War, exemplify selfless public service.
“Throughout his time as sheriff, Arpaio continued his life’s work of protecting the public from the scourges of crime and illegal immigration.
“Sheriff Joe Arpaio is now 85 years old, and after more than 50 years of admirable service to our nation, he is a worthy candidate for a presidential pardon.”
Joe Arpaio, who styled himself as “America’s toughest sheriff”, rose to national prominence for his sweeps of undocumented immigrants in Hispanic communities, and for detaining Spanish-speakers under suspicion of being undocumented immigrants.
In July 2017, the former sheriff was found guilty of violating a 2011 order to stop detaining immigrants.
The decision to pardon the former policeman was condemned by Democrats and civil rights groups.
Democratic Senator Patrick Leahy said it was “disheartening that [the president] set the bar so very low for his first pardon”.
Phoenix Mayor Greg Stanton called the move a slap in the face for the Latino community and those who had been victimized.
Joe Arpaio, who was born in Springfield, Massachusetts, could have faced six months in jail at his sentencing in October.
He served in the US military before he became a police officer – where he quickly acquired a reputation for his anti-immigration stance and tough enforcement tactics.
Joe Arpaio lost a bid for re-election in Arizona’s Maricopa County in November 2016, after 24 years in office.
He famously forced the prisoners to wear pink underwear and socks and old-fashioned black-and-white striped prison jumpsuits. The inmates lived outdoors while enduring sweltering Arizona desert temperatures.
Joe Arpaio also revived chain gangs, including a voluntary one for female prisoners.
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