JULY 18: His PTI party won 15 of 20 seats up for grabs in Punjab, beating their arch-rivals the PML-N on their home ground.
The result is being seen as a foretaste of what could happen in a general election due by October 2023.
Mr Khan was ousted as prime minister in a no-confidence vote in April.
The result in Punjab is a major blow for current Prime Minister Shehbaz Sharif, who leads the PML-N, and his coalition government in power nationally.
The party won just four of the seats holding by-elections, with one going to an independent candidate.
Punjab, Pakistan's most populous province, has long been a bastion of support for the PML-N of Mr Sharif, and his older brother, three-time former PM Nawaz Sharif.
Mr Khan has attracted thousands of Pakistan Tehreek-e-Insaf (PTI) supporters to rallies since being ousted - taking Punjab shows he is still a major electoral force to be reckoned with.
"The only way forward from here is to hold free and transparent elections," the former cricket star tweeted on Monday.
"Any other way will only lead to increased political uncertainty and further economic chaos."
Many media reports in Pakistan suggest the voters in Punjab wanted to send a message to the country's leaders about the economic hardships they are facing.
Prices are soaring as the government tries to tackle a foreign debt crisis, inherited in large part from Imran Khan's administration.
Mr Khan blames a US-led "foreign conspiracy" for his removal as prime minister. Washington has denied the claim.
