Mr Khan was found guilty of not declaring money earned from selling gifts he received in office. He denies the charges and says he will appeal.
After the verdict, Mr Khan was taken into custody from his home in Lahore.
In a pre-recorded statement posted after the verdict, he urged supporters to fight against the ruling.
"I have only one appeal, don't sit at home silently," he said in a video address posted to X, formerly known as Twitter. "I am struggling for you and the country and your children's future," he added.
The former cricketer-turned-politician, 70, was elected in 2018, but was ousted in a no-confidence vote last year after falling out with Pakistan's powerful military.
Mr Khan is facing more than 100 cases brought against him since his removal - charges he says are politically motivated.
However, the government has adamantly denied that there was any political motivation in Mr Khan's arrest or disqualification. Marriyum Aurangzeb, Pakistan's minister of information and broadcasting, told the BBC: "You have to be accountable for your deeds in law. This has nothing to do with politics. A person who has been proven guilty by the court has to be arrested."
Saturday's verdict centred on charges that Mr Khan incorrectly declared details of presents from foreign dignitaries and proceeds from their alleged sale.
The gifts - reported to be worth more than 140m Pakistani rupees ($635,000; £500,000) - included Rolex watches, a ring and a pair of cuff links.
"His dishonesty has been established beyond doubt," Judge Humayun Dilawar wrote in his ruling. Outside the courthouse, some pro-government demonstrators chanted "your show is over Khan".
