Chief Magistrate Victor So on Thursday initially 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 slogan was chanted on the streets during the months of pro-democracy demonstrations that rocked Hong Kong in 2019.
Chu was arrested on June 12 at a subway station wearing the shirt and a yellow mask printed with "FDNOL" - shorthand for 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 "Liberate Hong Kong" slogan called for the return of Hong Kong to British rule and he wore it to remind people of the protests, the court heard.
Slogans used during past protests have landed two men in jail under new national security laws. (AP PHOTO)
Magistrate So later sentenced another Hong Kong man, Chung Man-kit, 29, to 10 months in prison in a separate security case on Thursday.
"The content of the seditious words in this case involves advocating that the Hong Kong government to break away from the legitimate control of the central government and implement his ideas in a way that undermines social tranquility and public order," So said.
The law must intervene otherwise there was a risk of society "falling into chaos again," So said.
Chung on Thursday pleaded guilty to three counts of seditious acts for writing slogans advocating Hong Kong independence and "Liberate Hong Kong, revolution of our times" on the back of bus seats in March and April.
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 was 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 said they were needed to maintain stability.
Responding to Chu's sentencing, Amnesty International's China director Sarah Brooks said in a statement: "This is a blatant attack on the right to freedom of expression."
Amnesty urged Hong Kong authorities to repeal the law.
