Abdul Qadir Gilani won a by-poll in the family's traditional stronghold of Multan by bagging 64,628 votes, according to media reports.
His closest rival, Shaukat Bosan, an independent candidate backed by parties like the PML-N and Pakistan Tehrik-e-Insaf, secured 60,532 votes in the closely contested by-poll.
Observers said the victory was significant as it came in the wake of the former premier's conviction and disqualification by the apex court for refusing to reopen graft cases against President Asif Ali Zardari as well as growing disenchantment with the PPP-led government at the centre following widespread allegations of corruption.
However, they pointed out that Gilani's son had won by a margin of just over 4,000 votes an indication of the support extended to Bosan by "anti-PPP forces" like the Pakistan Tehrik-e-Insaf led by Imran Khan and the PML-N, which rules Punjab.
"Today the people gave their verdict. They have given a befitting reply to those who had sent my father packing. The anti-democratic forces should now stop conspiring against the PPP as it still the most popular party of the country," Abdul Qadir Gilani said.
During a three-week campaign, the Gilanis targeted the judiciary more than other political parties. Abdul Qadir said, "The people today have made it clear that they did not like its (judiciary's) decisions."
PPP insiders were of the view that Friday's by-poll was more a contest between the PPP and the superior judiciary led by Chief Justice Iftikhar Chaudhry and right wing parties.
No other mainstream political party had fielded a candidate against Gilani's son but all of them, including the PML-N and religious parties like Jamaat-e-Islami, supported Bosan, the brother of senior Pakistan Tehrik-e-Insaf leader Sikandar Bosan.
Bosan's election billboards and posters prominently features photos of Imran Khan, Nawaz Sharif, Punjab Chief Minister Shahbaz Sharif and Jamaat-e-Islami chief Munawar Hasan evidence of the support he enjoyed from these leaders.
"We are supporting Bosan as we want him and his brother to join the PML-N and contest on our party's platform," PML-N spokesman Pervaiz Rashid said.
Though Imran Khan announced that he would not forge an alliance with any other party, observers said the PML-N and Tehrik-e-Insaf were virtually in an alliance for the by-poll.