Centre-back Diallo joined Dortmund from Mainz last year in a deal said to be worth €28m and made 28 Bundesliga appearances as Lucien Favre's men finished second, but it looks set to be his only campaign at the club.
A Monaco youth product, Diallo had been strongly linked with a return to France with Lyon, but PSG have moved to the front of the queue since their interest became known.
The 23-year-old, a former France Under-21 international, had enjoyed a solid campaign with Dortmund, but Zorc accepts the deal suits them as much as the player.
When asked by Ruhr Nachrichten if Diallo was departing, Zorc said: "That has turned out so.
"Abdou has decided that he wants to take on another challenge and for us it was a good fit, so we agreed to the change."
Diallo will further bolster PSG's centre-back options as the club look to the future, with the Frenchman joining Marquinhos, Thilo Kehrer and Presnel Kimpembe as central defensive options who are all 25 or younger.
After Pablo Sarabia and Ander Herrera, Diallo will be the third new addition to PSG's first-team squad for next season.