A man who wore a T-shirt and mask bearing protest slogans has become the first person to be sentenced under Hong Kong's homegrown national security law, getting 14 months behind bars for sedition.

Chu Kai-pong, 27, earlier pleaded guilty to "doing an act or acts with seditious intent" under the Safeguarding National Security Ordinance.

He was arrested on June 12 near Shek Mun MTR Station for wearing a T-shirt and a yellow mask bearing the protest slogans.

Passing sentence at West Kowloon Court on Thursday, Chief Magistrate Victor So said the slogans were frequently used by protesters in 2019 and Chu had intentionally used them on a "symbolic day" to incite hatred towards authorities.

"June 12 is seen as a symbolic day by protesters. The road blockades that occurred in Admiralty and Wan Chai on that day [in 2019] kicked off a series of unrest, hitting society with riots and resulting in the destruction of a large number of public facilities and an imbalance of social order," So wrote in a judgement.

"The defendant used a symbolic day to incite others to commemorate the unrest and attempted to reignite the idea of unrest, which posed a huge risk to social order."

So noted that Chu had previously been jailed for three months for wearing a similar T-shirt at the airport, saying he showed no remorse by committing the same offence again.