The army released Pheu Thai party member Ruangkrai Leekijwattana at 9am on Saturday after a five-day detention for attitude adjustment.
A high-ranking army officer told Khaosod Online on Saturday Mr Ruangkrai was free to go after he was invited to stay at the 11th Military Circle in Bangkok on Tuesday.
Mr Ruangkrai earlier submitted a letter to US charge d'affaires Patrick Murphy, explaining his views on former PM Yingluck Shinawatra's impeachment and criticising the government. The junta was reportedly "very displeased" with his move.
- Earlier report: US 'deeply troubled' by detention
"We've come to an understanding and Mr Ruangkrai agreed to be more careful when expressing his political opinions," said the officer, who was not named.
"We didn't take legal action against him," he added.
Mr Ruangkrai's detention raised concern since most people summonsed for such sessions in recent months are usually released on the same day.
The US State Department said it was deeply troubled by the detention of dissidents in Thailand, AP reported.