Neither PSG nor Frankfurt disclosed the financial details of the deal. The Ecuador international, capped 16 times, becomes the first Ecuadorian player in PSG's history.
Having started his career at Ecuador's Independiente del Valle, Pacho made 44 appearances across all competitions for Frankfurt since joining from Belgian Pro League club Royal Antwerp last year.
"Willian is a fantastic young international player - and the latest top talent to join our club. At PSG we continue to build our project based on an amazing, young talented team - and most importantly, the collective team," PSG President Nasser Al-Khelaifi said in a statement.
On Monday, PSG signed highly rated 19-year-old Portuguese defensive midfielder Joao Neves from Primeira Liga side Benfica on a five-year contract.