It comes just days before a general election.
The special court found Khan guilty of making public the contents of a secret cable from Pakistan's ambassador in Washington to the Islamabad government - according to his party.
Former foreign minister Shah Mehmood Qureshi was also sentenced to 10 years in the same case.
Khan's party plans to challenge the ruling.
While officials with Khan's PTI party have called for calm... at least four activists were arrested outside a Karachi court on Tuesday, local media said.
A bomb in Pakistan's Balochistan region killed three PTI members on Tuesday, the party said, hours after Khan was sentenced.
The former prime minister has been in jail since last August.
That's when Khan was sentenced to three years in jail in a corruption case.
He's been fighting dozens of cases since he was ousted from power in a parliamentary no-confidence vote about two years ago.
