The sentence was handed down by a court established under the Official Secrets Act in Rawalpindi, a representative of his Pakistan Tehreek-e-Insaf (PTI) party said.
Former foreign minister Shah Mahmood Qureshi was also sentenced to 10 years in prison.
The PTI said it would challenge the decision and called it a “sham case”.
Khan's lawyer Salman Safdar condemned the verdict.
“This trial concluded in the most bizarre manner violating all norms of fair trial and procedure,“ he told The National.
Mr Safdar said he expected the verdict to be overturned once it reached higher courts.
Lawyer Ali Gohar Durrani, who is also representing PTI officials in court cases, told The National that the decision was illegal and unjust.
“Today's decision is not a legitimate court ruling but rather the judge's own will,” he said. “An appeal will be filed within 30 days, and a decision will be made promptly because it lacks legality.”
Khan was jailed last August after being convicted of corruption charges involving the personal sale of state gifts.
The latest case, known popularly as the "cipher case," centred around a classified diplomatic document authorities said he made public and displayed at an election rally.
The cable, from Pakistan's ambassador to Washington, alleges his ousting was a US-backed conspiracy, according to Khan.
Khan says that the contents of the cable had previously appeared in the media from other sources.
Political commentator Adil Shahzeb, who hosts a current affairs show on Pakistan's Dawn News platform, said he thought that while the PTI did use the classified cable to create a conspiracy around US involvement, the situation had been exploited for political gain.
“The trial should have been more transparent. Ten year's imprisonment is too harsh and that too [a] few days before elections,” Mr Shahzeb told The National.
The verdict was handed down about a week before Pakistan's parliamentary elections, from which Khan is barred for five years due to his previous criminal conviction.
Khan's PTI won the highest number of seats in the last elections in 2018, defeating the rival Pakistan Muslim League (Nawaz) and the Pakistan Peoples Party.
He was ousted as prime minister after a no-confidence vote in April 2022. In November of that year, he was shot and wounded in what he called “assassination attempt” while leading an anti-government rally to Islamabad.
Supporters of the PTI founder have demonstrated across the country since his arrest last May.
Authorities have clamped down on the PTI since then, banning party election rallies and detaining dozens of people at demonstrations.
About 2,000 gathered in Karachi on Sunday, where AFP correspondents saw about two dozen PTI supporters detained by police and taken away.
Pakistan's high court suspended his corruption conviction in late August and granted him bail in December, but he remained in jail on other charges.
Khan said he faced many charges – about 200 in all – including treason and terrorism.
His supporters and the PTI said the charges were politically motivated or manufactured by Pakistan's military and political elite, which they claim had decided to support the rival PML-N in the coming election.
Taimoor Saleem Jhagra, a PTI official and Vice President of Khyber Pakhtunkhwa province, where the PTI ruled for over 10 years, said that all the cases against Khan were rigged and had exposed Pakistan's justice system.
“[The case] is actually exposing how the Pakistani justice and governance systems don't work and how there is a concerted attempt to ensure that the people of Pakistan don’t have democratic right to vote Imran Khan to power,” he told The National.
He predicted that Pakistanis would vote for the PTI “in record numbers” on February 8.
A veteran Pakistani journalist said that the PTI would probably use the “victim card” in the election, rather than choose to boycott it.
“The upcoming election will likely be the most controversial in the history of Pakistan,” Lehaz Ali told The National.
“These types of decisions weaken the political structure of Pakistan, as they only target politicians, not the military, establishment, and bureaucracy.”