The 36-year-old reggae star better known as Bobi Wine had spent Friday holed up in a government hospital after being re-arrested while trying to leave the country.
Kyagulanyi became an MP in 2017, firing up a youthful population and proving a thorn in the side of ageing President Yoweri Museveni.
He was charged with treason last week after protesters stoned Museveni’s car during a by-election campaign.
The lawmaker’s arrest sparked violent protests and international condemnation over his alleged torture.
“He left last night at around 11.OO pm aboard KLM flight for Schipol (Amsterdam) onward to United States,” lawyer Nicholas Opiyo said.
“He was in the company of his wife, Barbie and brother Daks Sentamu.”
After being released on bail on Monday, Kyagulanyi was seized again by police on Thursday evening at Entebbe airport, outside the capital Kampala, his lawyers said.
Supporters say he was badly beaten and tortured while in army custody, during and after his arrest in the northwestern town of Arua earlier this month, and requires medical treatment abroad.
On Friday, Ugandan chief justice Bart Katureebe issued a public reminder that the constitution bans torture.
“Security forces, when you are arresting Ugandans, arrest them like human beings not animals,” he said.
Kyagulanyi, who chanted lyrics about poverty and social justice, has become a lightning rod for youthful opposition to the 74-year-old president.
Museveni, who has held power since 1986, is expected to seek another term in 2021 after constitutional changes were passed last year to scrape age limits.
Wine’s “age, his background and his story” make him a challenge unlike any Museveni has faced during his 32-year rule, said political analyst Rosebell Kagumire.