Lee Cheuk-yan, 69, and Chow Hang-tung, 41, were part of a group that organised annual vigils marking Beijing's 1989 Tiananmen Square crackdown on pro-democracy protesters.
Public commemorations of the 1989 protests used to be legal and tolerated in Hong Kong and were seen as a symbol of the city's relative freedoms compared to mainland China.
They were effectively outlawed in 2020 when the National Security Law came into force following mass protests the year before.
Lee and Chow, the former chair and former vice-chair of the now-disbanded Hong Kong Alliance in Support of Patriotic Democratic Movements of China, have been in custody since 2021.
Lee was sentenced to seven years, while Chow was sentenced to seven years and three months after being convicted in August under the national security law.
They had pleaded not guilty.
Another former chairman of the group and also a former chairman of the city's largest opposition Democratic Party, Albert Ho, 74, pleaded guilty in January.
He was sentenced to five years and two months in jail.
The Hong Kong Alliance faces a fine of HK$1.5 million ($A267,000), the court judgement said.
Lawyers for Lee, Chow and Ho were not immediately available for comment on Friday.
Prosecutors had argued the primacy of the Communist Party was enshrined in Article 1 of the Chinese constitution. Defence submissions that the defendants genuinely believed they were acting within the law given Hong Kong's freedoms of speech were rejected by the judges.
"By severely punishing the activists for conducting ordinary democratic activities, the Hong Kong High Court has violated their rights to freedom of expression and peaceful assembly," said Elaine Pearson, Asia director at Human Rights Watch.
