Yingluck Shinawatra, who is the younger sister of ousted Prime Minister Thaksin Shinawatra, is set to become the Thai leader just two months into her political career.
The election result is being seen as a victory for the rural, disaffected red-shirted protesters who clashed with the army last year.
Television footage showed Ms Yingluck, 44, swarmed by flashing cameras and journalists after exit polls indicated her Puea Thai (For Thais) party claimed a clear majority of the 500 seats in parliament.
With nearly all votes counted, her party won a projected 261 seats with current PM Abhisit Vejjajiva’s Democrat Party taking 162, the Election Commission said.
Ms Yingluck, a business executive, said: “I’ll do my best and will not disappoint you,” as she addressed supporters in Bangkok after receiving a phone call from her brother congratulating her.
Two months ago, Ms Yingluck, 44, was just another low-profile businesswoman with a husband and young son, but circumstances and the peculiar dynamics of the Thai power equation have projected her into the centre of the country’s political cauldron.
Prime Minister Abhisit Vejjajiva has conceded defeat. Projections from partial returns suggest that the Opposition Pheu Thai Party led by Thaksin’s sister Yingluck Shinawatra will win more than half the seats in Parliament to become Thailand’s first female Prime Minister.