The 26-year-old Krychowiak, a member of the Poland team who were eliminated by Portugal in the quarter-finals of the European Championship in France.
He joins PSG from Spanish side Sevilla, where he played under the guidance of Unai Emery who has now been appointed PSG coach.
PSG did not disclose the transfer fee, which was expected to reach around 25 million euros ($27.9 million), according to Polish and Spanish media.
Krychowiak started his career at the youth academy of Girondins Bordeaux and played for French clubs Stade Reims and Nantes before joining Sevilla in 2014, winning two successive Europa League titles with the Spanish club.
"I am very proud to return to France and defend the colours of a club as ambitious as Paris St Germain," Krychowiak said in a statement.
"The opportunity to continue working with Unai Emery was obviously a major factor in my decision," he added.
Krychowiak becomes PSG's second major signing under Emery after attacking midfielder Hatem Ben Arfa, who has joined from Ligue 1 side Nice on a two-year contract.
Emery has replaced Laurent Blanc, who parted company with PSG after three seasons in charge.
The ambitious and wealthy Paris club achieved their second successive domestic treble by winning Ligue 1, the French Cup and the League Cup, but they lost in the Champions League quarter-finals for the fourth successive season.
($1 = 0.8978 euros)
(Reporting by Patrick Vignal, editing by Ed Osmond)
