The Polish Press Agency is Poland's largest provider of both domestic and international news.
The move, reported by independent outlet Zerkalo.io, followed a ruling by a district court in the Mahilyou region on July 28.
Tsikhanouskaya said the listing was recognition of PAP's work in spreading reliable and independent information, adding that it showed the agency's reporting was reaching Belarusians and that "the regime is once again showing its weakness and absurdity."
Her adviser, Franak Viacorka, called it a classic tactic of totalitarian regimes, aimed at intimidating society, cutting people off from independent information and identifying "enemies of the state."
He noted that Belarus now brands not just politicians and activists as "extremists" but also independent media, civic groups, artists, academics and people who simply help others.
Viacorka warned the designation amounted to a form of repression against ordinary Belarusians: anyone reading, sharing or subscribing to PAP's social media profiles could now face accusations of extremist activity and criminal liability.
He added that Belarusian authorities apply no consistent logic in deciding who ends up on such lists, and that PAP joins organisations including the BBC and the European Humanities University.
PAP's editor-in-chief Marek Błoński said the agency viewed the court's decision as an attempt to restrict access to reliable, independent information.
He stressed that PAP would continue reporting on Belarus in line with professional journalistic standards while taking particular care over the safety of its sources and audiences there.
Belarus has maintained its register of "extremist" content since 2008.
As of February this year, the country's state media reported more than 9,000 entries on the list, covering social media profiles, books, websites of international organisations and sites listing political prisoners.
Viasna, a leading Belarusian human rights organisation, said last autumn that being listed brings administrative liability not only for site owners but also for anyone who comments on, likes, reprints or shares the material.
Penalties range from fines to administrative detention, and, in theory, criminal charges for "supporting extremist activity."
(ał)
Source: PAP