Mr. Wilson was convicted of a felony charge of aggravated child molestation in 2005 for the consensual sex act with a 15-year-old girl while he was 17. The Legislature has since changed the law so that situation would be a misdemeanor.
"I was in total disbelief," the 21-year-old told the Associated Press outside the prison. "I'm finally happy to see we've got justice now."
Civil rights leaders and Georgia lawmakers who have rallied behind Mr. Wilson lauded the ruling.
"He is a symbol of what's gone wrong, and what could go right," said the Rev. Jesse Jackson, the president of the Rainbow/PUSH Coalition who was in Atlanta for a group conference. "Something about Genarlow Wilson touched the conscious of our nation."
Mr. Wilson's case drew widespread attention as he fought to have his sentence reversed.
The charges stemmed from a 2003 party at a hotel in Douglas County. A jury found him guilty of the child molestation based on a videotape showing him with the girl, who was under the legal age.