The party, which rode to power in January by vowing to end austerity measures, was deeply divided over Greece's acceptance of a third international bailout package.
The revolt within his own party prompted Tsipras to announce his resignation late on Thursday, unleashing early elections which are likely to take place on September 20, state news agency ANA said, citing government sources.
The new party, called Popular Unity, will be led by former energy minister Panagiotis Lafazanis, 63, according to the news website left.gr.
Lafazanis, removed from his post in a cabinet reshuffle last month, is an implacable foe of the agreement reached between Athens and the European Union for a 86-billion-euro ($96-billion) bailout package, the first tranche of which was released on Thursday.