U.S. District Judge Susan Bolton said Arpaio is guilty of contempt of court for failing to comply with an order from U.S. District Judge G. Murray Snow in 2011 to stop detaining people based only on the belief that they are in the United States illegally.
Arpaio, 85, the Republican sheriff of Maricopa County, Ariz., for 24 years, was a staunch opponent of illegal immigration and instituted policies that led some supporters to refer to him as the "toughest sheriff in America."
Arpaio's lawyers argued that Snow's order was unclear and that willful intention was required to prove criminal contempt.
The former sheriff issued a statement to the Arizona Republic, saying he plans to appeal the ruling so that he may get a trial by jury.
"Today, Judge Susan Bolton violated the United States Constitution by issuing her verdict without even reading it to the defendant in public court," the statement said. "Her verdict is contrary to what every single witness testified in the case. Arpaio believes that a jury would have found in his favor, and that it will."
Bolton scheduled sentencing to take place Oct. 5.
Doug G. Ware contributed to this report.