The French club gave no details of the transfer but media reports suggest the Ligue 1 title-chasers have parted with 10million euros (£8.3million) to secure the services of the 29-year-old.
"A press conference will be given on Wednesday February 1 2012 at 15h30 at the Parc des Princes to present Thiago Motta," read a statement on official club website psg.fr.
"This presentation will be attended by PSG's sporting director, Leonardo."
The Brazil-born Motta, who will wear the number 28 shirt at PSG, had a contract with the Nerazzurri until June 2013. He joined Inter in 2009 from Genoa and scored three goals in 10 league appearances this season.
Claudio Ranieri had included Motta in his squad for Wednesday's Serie A match against Palermo, and the departure of the Italy international will come as a blow to the Inter coach, who had been given guarantees that the player would remain until the summer.
"I requested the club for Motta to stay and he will stay," Ranieri said earlier this week.
"The club has assured me that he will remain at least until June."
However, the club this afternoon flew in the face of Ranieri's claim as they confirmed Motta's departure.
"Inter announces it has transferred Thiago Motta to Paris St Germain," a statement read.
"We wish Motta the best of luck in his career in France with PSG."
There was better news elsewhere on the transfer front for Ranieri, however, after Inter announced they had strengthened their midfield line with the arrivals of Angelo Palombo from Sampdoria and Fredy Guarin from Porto.
Both have joined on loan until the end of the season, with the Nerazzurri having an option to make their stays permanent in the summer.
The club also confirmed that Brazilian defender Juan Jesus has signed a contract until June 2016. The Brazil Under-20 international arrived earlier this week on a permanent deal from Internacional de Porto Alegre.