In his first act of presidential clemency, Trump pardoned the deeply-divisive 85-year-old who ignored a federal court order that he stop detaining illegal migrants.
"He kept Arizona safe!" Trump tweeted, calling Arpaio a "patriot."
I am pleased to inform you that I have just granted a full Pardon to 85 year old American patriot Sheriff Joe Arpaio. He kept Arizona safe!— Donald J. Trump (@realDonaldTrump) August 26, 2017
The move earned immediate scorn from Democrats, some Republicans and rights groups, who accused the Republican billionaire of seeking to divide the country -- which is still reeling from Trump's controversial remarks on racial unrest in Charlottesville.
In a statement, the White House said Arpaio -- who made detainees wear pink underwear and housed them in tented desert camps -- had "more than fifty years of admirable service to our nation."
The former sheriff of Maricopa County, who reveled in his reputation as "America's toughest sheriff," had been due to be sentenced in October.
Arpaio tweeted that he was "incredibly grateful" to Trump, and suggested his conviction was "a political witch hunt by holdovers in the Obama justice department!"
I am humbled and incredibly grateful to President Trump. I look fwd to putting this chapter behind me and helping to #MAGA— Joe Arpaio (@RealSheriffJoe) August 26, 2017
Thank you @realdonaldtrump for seeing my conviction for what it is: a political witch hunt by holdovers in the Obama justice department!— Joe Arpaio (@RealSheriffJoe) August 26, 2017
He also asked supporters to donate to his legal defense fund.
'Worthy of pardon'
According to the White House, as sheriff Arpaio protected "the public from the scourges of crime and illegal immigration."
Arpaio is "a worthy candidate for a presidential pardon," it added.
Trump had hinted that a pardon was coming during a meandering speech in Arizona earlier this week, when he suggested Arpaio was convicted for "doing his job" and predicted that "he's going to be just fine."
The announcement still came as a shock for many.
Republican Arizona Senator John McCain said officers of the law "should always seek to be beyond reproach" in their commitment to fairly enforce the law.
And he noted that Arpaio "was found guilty of criminal contempt" for illegally profiling Latinos living in Arizona "based on their perceived immigration status in violation of a judge's orders."
Trump "has the authority to make this pardon, but doing so at this time undermines his claim for the respect of rule of law as Mr. Arpaio has shown no remorse for his actions," McCain's statement read.
Arizona's other senator, Republican Jeff Flake, wrote on Twitter: "Regarding the Arpaio pardon, I would have preferred that the President honor the judicial process and let it take its course."
'Presidential endorsement of racism'
Others were more forceful in their denunciations.
"Joe Arpaio is a bigot who targeted the Hispanic community for years. He should have served his time," said Democratic Congressman Joaquin Castro.
The American Civil Liberties Union, a leading civil rights group, expressed outrage.
"With his pardon of Arpaio, Trump has chosen lawlessness over justice, division over unity, hurt over healing," said the ACLU's deputy legal director Cecillia Wang.
"Once again, the president has acted in support of illegal, failed immigration enforcement practices that target people of color and have been struck down by the courts.
"His pardon of Arpaio is a presidential endorsement of racism."
UnidosUS, the largest Hispanic rights advocacy group in the country, decried the pardon as "obscene."
"Sheriff Joe Arpaio was the instigator of racial profiling and made official a policy of harassment and abuse based on the color of one's skin in Maricopa County," said UnidosUS president Janet Murguia.
"And tonight, president Trump gave the blessing of his administration to pursue those disgraceful and unlawful policies in every state and locality in the land."
The US constitution offers the president almost unlimited pardon powers.
Conservative commentator Bill Kristol suggested the pardon "gets people used" to the idea of presidential pardons as the FBI investigation into the Trump campaign's alleged collusion with Russia deepens.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)