Joshua Wong, Nathan Law and Alex Chow were jailed in August last year for their role in the 2014 Umbrella Movement mass pro-democracy protests, after Hong Kong's government pushed for harsher sentences.
A lower court had given Wong and Law community service orders and Chow a suspended sentence. But after the government's intervention they were jailed for between six and eight months by the Court of Appeal.
All three activists were later bailed pending their appeal.
Chief justice Geoffrey Ma said Tuesday that the terms given to the trio were "significantly more severe" than the range previously handed down for unlawful assembly offences.
Handing down the judgement in the Court of Final Appeal he also said had been "inappropriate" to hand the sentences down retrospectively.
The government's move to seek jail sentences for the activists was seen as further evidence of Beijing's growing influence over the semi-autonomous city, with Chinese authorities particularly riled by the emergence of activists calling for independence for Hong Kong.
Wong and Law's political party Demosisto wants self-determination for the city.//AFP