The by-elections come after a national vote on July 25 propelled Imran Khan to power.
Pakistan's ruling party under new Prime Minister Imran Khan has maintained its slim majority in parliament after key by-elections, the final results showed on Monday.
Mr Khan's Pakistan Tehreek-e-Insaf (PTI) won 15 seats of the total 36 on offer in polls held across the country on Sunday, according to the Election Commission of Pakistan.
Its ally the Pakistan Muslim League, one of several parties with which it has formed a coalition government, won an additional two seats, bringing the coalition's total in the national assembly -- the lower house of parliament -- to 177 out of 342.
The main opposition Pakistan Muslim League-Nawaz (PML-N) increased its seat count to 85.
The by-elections come after a national vote on July 25 propelled former World Cup cricketer Khan to power, an outcome that was rejected by the main opposition parties.
Most of the national assembly seats had been open as Pakistan allows candidates to run in multiple constituencies, but keep only one seat.
The general election -- dubbed "Pakistan's dirtiest" -- had been criticised by the United States, the European Union and other observers after widespread claims that the powerful military was trying to fix the playing field in Mr Khan's favour.