Outgoing premier Abhisit Vejjajiva's Democrats, who have already conceded defeat, took 159 seats, the poll body said.
It said turnout was almost 75 percent, roughly the same as in the previous election in 2007.
The Election Commission is, however, studying some complaints of possible irregularities, which could see candidates given "red cards" and disqualified, potentially whittling down Puea Thai's majority.
The result is a remarkable comeback for Thaksin's supporters after his ouster in a military coup sparked years of turmoil, and his youngest sister Yingluck Shinawatra is set to become Thailand's first female premier.
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