In exchange for the release of political prisoners, the US said it would relieve some sanctions on Belarusian airline Belavia, allowing the carrier to buy parts for its planes.
Stephen Doughty called the release “a significant breakthrough” and thanked the US for “substantial diplomatic efforts to secure this outcome”.
The prisoners released included trade union leaders, journalists and activists, but more than 1,000 political prisoners remain in jail in Belarus.
Julia Fenner had previously worked at the British embassy in Minsk before marrying British diplomat Martin Fenner, according to human rights group Charter 97. Martin Fenner was deputy head of mission in Minsk for four years in the early 2000s.
Another rights group, Spring 96, recognised Mrs Fenner as a political prisoner , externalwho had been imprisoned in a penal colony.
Although the reason for Belarus charging her was never explained, she was accused under two articles of the criminal code, for active participation in actions that grossly violate public order and assistance to extremist activity.
All opposition has been quashed by Belarusian authoritarian leader Lukashenko, 71, who has been in power since 1994.
He described the release of the 52 prisoners as a humanitarian gesture, after meeting Donald Trump’s special envoy John Coale in the capital Minsk on Thursday.
In a news conference on Friday, Belarus’s exiled opposition leader Svetlana Tikhanovskaya thanked the US president for securing the prisoners’ release.