Judge Esther Toh said 18 years of Lai's sentence should be served consecutively to his jail term in his fraud case, for which he received a sentence of five years and nine months.
Lai can appeal his case. His co-defendants received jail terms between six years and three months and 10 years.
Three government-vetted judges spared the 78-year-old Lai the maximum penalty of life imprisonment on charges of conspiring with others to collude with foreign forces to endanger national security, and conspiracy to publish seditious articles.
He was convicted in December. Given his age, the prison term still could keep him behind bars for the rest of his life.
Before Lai left the courtroom on Monday, he looked serious, as some people in the public gallery cried.
Hong Kong's outspoken Catholic Cardinal Joseph Zen sat next to Lai's wife earlier when he arrived for the sentencing.
The democracy advocate's arrest and trial have raised concerns about the decline of press freedom in what was once an Asian bastion of media independence.
The government insists the case has nothing to do with a free press, saying the defendants used news reporting as a pretext for years to commit acts that harmed China and Hong Kong.
Lai was one of the first prominent figures to be arrested under the security law in 2020.
Within a year, some of Apple Daily's senior journalists also were arrested.
Police raids, prosecutions and a freeze of its assets forced the newspaper's closure in June 2021. The final edition sold a million copies.
Lai's sentencing could heighten Beijing's diplomatic tensions with foreign governments. His conviction has drawn criticism from the US and the UK.
US President Donald Trump said he felt "so badly" after the verdict and noted he spoke to Chinese leader Xi Jinping about Lai and "asked to consider his release".
British Prime Minister Keir Starmer's government also has called for the release of Lai, who is a British citizen.
Lai's daughter, Claire, told The Associated Press that she hopes authorities see the wisdom in releasing her father, a Catholic. She said their faith rests in God.
"We will never stop fighting until he is free," she said.
Judges ruled Lai was the mastermind
Lai founded Apple Daily, a now-defunct newspaper known for its critical reports against the governments in Hong Kong and Beijing.
He was arrested in August 2020 under the security law that was used in a years-long crackdown on many of Hong Kong's leading activists.
During his 156-day trial, prosecutors accused him of conspiring with six former Apple Daily staffers, two activists and others to request foreign forces to impose sanctions or blockades or engage in other hostile activities against Hong Kong or China. Lai testified for 52 days in his own defence, arguing he had not called for foreign sanctions after the law's introduction.
In December, the judges ruled Lai was the mastermind of the conspiracies and never wavered in his intention to destabilise the ruling Chinese Communist Party.
They took issue with what they called his "constant invitation" to the United States to bring down the Chinese government with the excuse of helping Hong Kongers.
Lai is serving a nearly six-year prison term over fraud allegations in a separate case and has been in custody for more than five years.
In January, lawyer Robert Pang said Lai suffered health issues including heart palpitations, high blood pressure and diabetes.
Although Lai's condition was not life-threatening, Pang argued his client's health, age and solitary confinement, which the prosecution said Lai requested, would make his sentence "more burdensome".
The prosecution said a medical report noted Lai's general health condition remained stable.
Lai founded Apple Daily in 1995, two years before Hong Kong returned to Chinese rule after 156 years as a British colony.
The publication drew a strong following with reports that were occasionally sensational, investigative scoops and short, animated video reports.
Articles supporting the city's democracy movement, including anti-government protests that rocked the city in 2019, attracted many pro-democracy readers.
In 2022, Hong Kong plunged 68 places to 148th out of 180 territories in the press-freedom index compiled by media freedom organisation Reporters Without Borders. The city's latest ranking was 140th, far from 18th place in 2002.
