HONG KONG — A Hong Kong judge on Thursday jailed two men in the Chinese territory’s first sedition cases under a new national security law, one for wearing a shirt with a protest slogan and another for writing similar messages on bus seats.

Chief Magistrate Victor So 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…