The crime carries a maximum sentence of life in prison.
Wong, 29, rose to international prominence as a teenage democracy activist in Hong Kong, one of a group of youngsters whose protest efforts have now effectively been outlawed by a sweeping national security law that Beijing imposed on the Asian financial hub in 2020.
Wearing a dark blue shirt and black-rimmed glasses, Wong waved and nodded to people from the court's dock on Wednesday, inhaling deeply at times, before pleading guilty to a government-approved national security judge at Hong Kong's High Court in the district of Admiralty.
Wong is already in jail serving an existing sentence on an earlier national security charge of conspiracy to commit subversion after he was among a group of democratic activists who participated in an unofficial primary election in 2020.
His current sentence is scheduled to end next year.
A charge sheet seen earlier by Reuters accused Wong of having conspired with exiled activist Nathan Law and others to ask foreign countries, institutions, organisations, or individuals outside China to impose sanctions or blockades.
Such actions against Hong Kong or China, along with other hostile activities targeting them, took place in 2020, between July 1 and November 23, it added.
