Chief Magistrate Victor So on Thursday sentenced Chu Kai-pong, 27, to 14 months' jail under tougher sentencing laws created by the city's legislature in March.
"If the law does not intervene early and allows individual sedition to occur, it will eventually lead to chaos in the society again," So said in a written judgment.
Chu pleaded guilty plea to having seditiously worn a T-shirt with the protest slogan "Liberate Hong Kong, revolution of our times".
The message - chanted on streets during the months of pro-democracy protests that rocked Hong Kong in 2019 - had been deemed capable of inciting secession in a separate national security case.
Chu was arrested on June 12 at a subway station wearing the shirt and a yellow mask printed with "FDNOL"- the shorthand of another popular 2019 slogan "five demands, not one less".
The date marked the fifth anniversary of one of the first major actions in the protests.
"The defendant used a symbolic day to incite others to commemorate the unrest and try to revive the idea of the unrest, which poses a great risk to the social order, and the circumstances are not minor," So said.
Chu told police the slogan "Liberate Hong Kong, revolution of our times" called for the return of Hong Kong to British rule and he wore it to remind people of the protests, the court heard.
Chu's lawyer Steven Kwan on Monday told the court there was no evidence that people were affected or incited by the slogan in the 25 minutes Chu wore the T-shirt in public.
Beijing imposed a national security law in 2020 to quash the months-long protests in the financial hub. In March 2024, Hong Kong passed a second new security law.
Under the new law, the maximum sentence for sedition has been increased from two years to up to seven years in prison and up to 10 years if "collusion with foreign forces" is involved.
Critics, including Western governments, have said the vaguely defined provisions could be used to curb dissent but Hong Kong and Chinese officials have said it was needed to plug loopholes and maintain stability.
