PSG, who were bought by Qatari investors at the end of last season and have made a raft of new signings now totalling over 80 million euros, announced the deal on their Twitter feed but have not released financial details.
The previous French record buy was when PSG took Nicolas Anelka from Real Madrid for 33.5 million euros in 2000.
The signature of Pastore, who was being courted by some of Europe's top clubs, comes on the opening day of the French league season and he could be unveiled to the Parc des Princes fans before the game with Lorient later.
The 22-year-old, who takes the number 27, had announced a deal on Tuesday and arrived in Paris on Thursday but a contract took some time to iron out.
New sporting director Leonardo left as coach of mighty Inter Milan to come back to former club PSG and mastermind their bid for French and European domination thanks to their newfound wealth.
He has used his Italian contacts to bring in Pastore as well as Jeremy Menez from AS Roma, Mohamed Sissoko from Juventus and goalkeeper Salvatore Sirigu also from Palermo.
PSG have underachieved as a capital city side for much of their short history in a Ligue 1 outshone by other European top flights in terms of financial muscle.
