Imran Khan served nearly four years as PM before he was ousted in April
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 a foretaste of what could happen in a general election due by October 2023 but which could be sooner.
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. His already weak coalition government's fate now hangs by a thread.
Pakistan is reeling from unprecedented inflation and energy shortages - now political instability could spiral out of control.
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.
But the party won just four of the seats in Sunday's by-elections, with one going to an independent candidate.
The by-elections were called after MPs from the Pakistan Tehreek-e-Insaf (PTI) were disqualified for switching allegiance in a vote to elect Mr Sharif's son Hamza as Punjab chief minister. His short term in office now looks set to end.
But Imran Khan has the wind in his sails. He has been attracting thousands of PTI supporters to rallies since he was ousted.
"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."
Analyst Cyril Almeida says since it became clear to Mr Khan he was going to lose the vote of confidence in parliament, he has had a one point agenda: fresh elections as soon as possible.
"Now it's within his grasp," Mr Almeida says. "They may try and limp on…but the government is now effectively at Imran's mercy."
Fuel prices have rocketed in recent weeks, hitting people's pockets hard
The result in Punjab suggests voters there 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 before he was ousted.
It was the first time in Pakistan's history a sitting prime minister had lost a vote of confidence. Mr Khan blames a US-led "foreign conspiracy" for his removal. Washington has denied the claim.
Journalist and analyst Benazir Shah says there was already a camp within the governing party which wanted early elections, but they were sidelined.
"We could again hear those voices within the PML-N, pushing for an immediate election. However, perhaps none of the allied parties with the PML-N would agree to an election before October 2023."
Ms Shah believes the Punjab result clearly shows the PTI is in the ascendency.
"Imran Khan was able to secure this victory for his party without the help of the military; without businessmen, who previously funded his electoral campaigns, and without having any major electable candidates [who have dependable vote banks] in his camp," she says.
But she also expressed caution about reading too much into these results and interpreting them as a bellwether for rest of the country.
"While the PTI's popularity is on a upward slope, a true referendum on where the PML-N stands in Punjab will only be possible in general polls."
What makes life even more complicated for the ruling coalition is Pakistan's precarious economic situation.
It was forced to introduce a steep hike in fuel prices and remove other subsidies within weeks of taking office, in order to meet International Monetary Fund (IMF) conditions to resume a $7.2bn aid package.
Cyril Almeida believes that the Punjab result has essentially "driven a stake through the IMF deal".
"Had the PML-N won, the government would have been looking towards hikes in the rates of electricity. Instead, the markets have dipped on the back of obvious uncertainty."
He says if Imran Khan's team get back into office, they may even seek to renegotiate a PML-N deal with the IMF that itself was a renegotiation of an existing PTI deal.
"The economy will remain in deep trouble," he says.
-- Courtesy of BBC News