Ma Chun-man is the second person to be tried under the controversial law and the first to be convicted for non-violent offenses.
A Hong Kong activist known as "Captain America" has been found guilty of inciting secession during pro-democracy protests.
Ma Chun-man is the second person to be tried under Hong Kong's National Security Law.
But he was the first to be convicted for non-violent crimes under the controversial law, which was imposed last year after widespread demonstrations.
The 31-year-old former food deliveryman has been on remand since his arrest ten months ago and pleaded not guilty. He faces seven years in prison and is due to be sentenced in November.
Ma was accused of rallying for Hong Kong's independence from China by chanting slogans and holding up placards during the protests.
He was dubbed online by many as "Captain America 2.0" for wielding the superhero soldiers' shield during protests.
Prosecutors say he chanted words such as "Free Hong Kong, the revolution of our time" and "Hong Kong independence is the only way out".
His lawyers had argued that the slogans were "empty words" and that he was simply trying to prove that "exercising freedom of expression is not against the law".
Beijing has said that the National Security Law has brought stability to Hong Kong after the 2019 unrest, punishing "secession" and "terrorism".
But pro-democracy activists say that the legislation has cracked down on all forms of dissent in Hong Kong. More than 100 people, including many prominent activists, have been charged under the law.
In July, Tong Ying-kit became the first person convicted under the law for riding a motorcycle into police while waving a flag with a protest slogan. He was sentenced to nine years in prison for charges of inciting secession and terrorism.
Meanwhile, the human rights group Amnesty International has announced that it will close its two offices in Hong Kong this year.
The NGO is the latest to cease operations in the former British colony amid the crackdown on political dissent.
"This decision, made with a heavy heart, has been driven by Hong Kong's national security law," said Anjhula Mya Singh Bais, chair of Amnesty International's board.
"[The law] has made it effectively impossible for human rights organisations in Hong Kong to work freely and without fear of serious reprisals from the government."