The vote ensured Europe's most outspoken leftist leader would remain Greece's dominant political figure, despite having been abandoned by party radicals last month after he caved in to demands for austerity to win a bailout from the euro zone.
In a victory speech to cheering crowds in a central Athens square, he promised a new phase of stability in a country that has held five general elections in six years, saying his mandate would now see him through a full term.
He made no specific reference to the 85 billion euro bailout, but Syriza campaigned on a pledge to implement it, while promising also to introduce measures to protect vulnerable groups from some aspects of the deal.
Tsipras's first task after forming a government will be to persuade European Union lenders that enough agreed steps have been made to ensure the next payment. The bailout program is due for a review next month.