Ryan attorney Jim Thompson, also a former Illinois governor, said the Bureau of Prison suspended a requirement that Ryan serve the next phase of his sentence at a halfway house in Chicago and ordered him instead to submit to house arrest at his home in Kankakee.
Ryan, 78, was sentenced to 6 1/2 years for corruption.
The Chicago Tribune reported Ryan arrived at the halfway house at 7 a.m. and was released a short time later.
"He would like me to tell you he's grateful to leave the penitentiary. He's grateful also for the encouragement and support from many people," Thompson said. "He has paid a severe price. The loss of his wife and brother while he was in the penitentiary, the loss of his pension, his office, his good name and 5 1/2 years of imprisonment. Now near 80 years old, that is a significant punishment. But he is going to go forward."
A federal jury convicted Ryan of corruption in 2006 stemming from bribery at drivers' license facilities that occurred while he was secretary of state.
Ryan entered federal prison in November 2007.
Ryan's successor as governor, Rod Blagojevich, is in the first year of a 14-year sentence for corruption.