French-trained lawyer Cu Huy Ha Vu, the son of a Vietnamese revolutionary leader, was sentenced in April 2011 to seven years in prison for "anti-state activity".
The release of the 55-year-old, who last year staged a hunger strike to draw attention to his treatment in jail, followed intense campaigning by rights groups and foreign governments.
"We welcome the decision by Vietnamese authorities to release prisoner of conscience Dr. Cu Huy Ha Vu," US Embassy spokesman Spencer Cryder told AFP.
"Dr. Vu and his wife decided to travel to the United States after his release from prison and arrived in Washington DC on Monday," he added.
He declined to say whether Vu would live permanently in the United States, but Washington has granted asylum to Vietnamese dissidents in the past.
The Vietnamese government has not said why it freed Vu, who according to his wife suffered from heart problems.
Vu's lawyer Tran Vu Hai said the reason for his release was unclear.
The veteran activist is the son of Cu Huy Can, who was a member of revered founding president Ho Chi Minh's provisional cabinet from 1945 and remains a celebrated poet.
Vu was arrested in 2010 after attempting unsuccessfully to sue Prime Minister Nguyen Tan Dung over a bauxite mining plan that sparked unusually broad opposition.
The head judge in his trial said Vu's writings and interviews "blackened" the Communist Party of Vietnam.
Former wartime foes Vietnam and the US have worked to improve ties in recent decades, but concerns over human rights have sometimes strained the relationship.
The one-party state is regularly denounced by rights groups and Western governments for its intolerance of political dissent and systematic violations of freedom of religion.
Human Rights Watch's Asia deputy director Phil Robertson said Vu's release was a "welcome development, especially given the serious health issues he developed while unjustly imprisoned by the Vietnam authorities."
"He should never have been tried or imprisoned in the first place because all he did was exercise his right to free speech," Robertson added.
Vietnam bans private media and all newspapers and television channels are state-run. Lawyers, bloggers and activists are regularly subject to arbitrary arrest and detention, according to rights groups.
Reporters Without Borders said earlier this month that Vietnam was second only to China in the number of bloggers it detained with at least 34 behind bars.