The Foreign Secretary announced the move because of the "systematic erosion of liberty and democracy" in the territory since it was handed back to China.
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Eight British judges will be withdrawn from the court of final appeal, set up in 1997 at the handover.
Two years ago China banned views being expressed on independence and restricted the media from criticising the regime, with people facing extradition to the mainland.
It sparked huge protests on Hong Kong and condemnation from the international community, with the UK offering refuge to thousands of people.
Ms Truss said: "We have seen a systematic erosion of liberty and democracy in Hong Kong. Since the national security law was imposed, authorities have cracked down on free speech, the free press and free association."
"The situation has reached a tipping point where it is no longer tenable for British judges to sit on Hong Kong's leading court, and would risk legitimising oppression.
"I wholeheartedly support the decision to withdraw judges."
Her statement was swiftly followed by the judges including Lord Robert Reed, president of the UK's supreme court, quitting. He said they could not "appear to endorse an administration which departed from political freedom and freedom of expression."
A Foreign Office source said: "Liz has strengthened our stance on China and will continue calling out its attempts to undermine freedom."