Hong Kong's top court has granted a bid by young democracy activist Joshua Wong to appeal his prison sentence.

Tuesday's decision gives Wong and fellow activist Nathan Law one last chance to fight their prison terms for involvement in an unlawful assembly that sparked huge 2014 pro-democracy protests in the Chinese-controlled city.

Wong got six months and Law received eight months in prison. They were bailed last month after serving two months so they could apply for an appeal, which the Court of Final Appeal approved.

The court scheduled their appeal hearing for January.

The 21-year-old Wong shot to stardom for his role helping spearhead the protests three years ago against Beijing's decisions to restrict elections for the city's top leader.