Emanuel Zeltser, a 55-year-old diabetic, was sentenced to three years in prison in August 2008 after being convicted in a closed trial on charges that his supporters called politically motivated.
In November, Zeltser was placed in a prison hospital after arriving at a penal colony in eastern Belarus where he was denied medicine, lawyers said.
On leaving the prison clinic in the eastern town of Mogilyov, Zeltser said Tuesday, "I am glad about my freedom.
"I have problems with my health, I plan to get better. I am not making any plans," he said by telephone outside the prison.
He refused to comment further.
Zeltser, slightly limping, was met by U.S. officials and entered a car with them to head for the U.S. Embassy in Minsk.
President Alexander Lukashenko signed a decree pardoning Zeltser earlier Tuesday.
Lukashenko is on a drive to court better political and economic ties with the West, and Washington had said Zeltser's release would help the process.
U.S. State Department spokesman Ian Kelly said of Zeltser's release, "The United States welcomes this positive step."
He added in a statement that consular officials were working with his family "to arrange his swift and safe return to the United States."
The Russian-born Zeltser is a high-profile lawyer who headed the nongovernmental American Russian Law Institute in New York. He is a renowned expert on organized crime and money laundering, particularly in former Soviet republics.
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