Thai television showed a frail and tired-looking Yettaw in a wheelchair and wearing a face mask being pushed through the airport.
Cynthia Brown, a spokeswoman for the US embassy in the Thai capital, said she could not comment on Yettaw's departure or travel plans citing "privacy concerns".
Yettaw was sentenced to seven years in jail with hard labour for swimming uninvited to the home of Suu Kyi
The military government said that the visit violated the terms of the Nobel peace laureate's house arrest.
The White House said it welcomed Myanmar's move to free Yettaw but also urged the military government "to release all the political prisoners it is holding in detention or in house arrest, including Aung San Suu Kyi".
But Suu Kyi, who was sentenced to a further 18 months of house arrest after Yettaw's visit, criticised the military government's double standards.
"The fact that the person who entered the house was released whereas residents of the house remained detained is ugly," Nyan Win, her lawyer, quoted her as saying on Monday.
Suu Kyi has been in detention for 14 of the last 20 years.
Myanmar state television said Yettaw, who was held for three months in a Myanmar jail during his joint trial with Aung San Suu Kyi, was freed on humanitarian grounds because of his poor health.
He suffers from diabetes and was hospitalised for a week during the trial after suffering seizures.
Phillip Crowley, a US state department spokesman, said Yettaw's release could not be seen as an indication that Myanmar, formerly known as Burma, was embracing change.
"We remain very concerned about the continued detainment of Aung San Suu Kyi and more than 2,100 prisoners that are in detention,'' he said.
"We'll be looking for signs that Burma's fundamentally changing its approach and its policies."