Wong, 28, was originally set to be released in January 2027 from a 56-month jail sentence he is serving under the same law for conspiracy to commit subversion after he participated in an unofficial primary election.
Taken to the West Kowloon magistrates' courts, Wong faced a new charge of conspiracy to collude with a foreign country or with external elements to endanger national security.
The former student pro-democracy activist, who wore a blue shirt and appeared noticeably thinner than before, replied, "Understand," when the clerk read out the charge and details of the offence.
Wong did not apply for bail, and the case was adjourned to August 8.Â
Before returning to custody, he waved, shrugged, and shook his head in the direction of the public gallery.
In a statement, Hong Kong's national security police said they had arrested a 28-year-old man on suspicion of the offence, as well as for "dealing with property known or believed to represent proceeds of an indictable offence".
A charge sheet seen by Reuters accuses 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.
The National Security Law, which punishes offences such as acts of subversion, collusion with foreign forces, and terrorism, with terms of up to life in jail, was imposed by Beijing on the former British colony in 2020.
The Chinese and Hong Kong governments say the law is necessary to restore stability following anti-government protests in 2019.
But some Western governments have criticised it as being used to suppress free speech and dissent.
