Gilani's son Abdul Qadir Gilani won in the family's traditional stronghold of Multan by bagging 64,628 votes, according to unofficial results flashed by the state-run media.
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 bye-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