Schaars, 29, has signed a three-year contract at the Philips Stadion with 21-year-old Arias following his Sporting team-mate to Holland on a four-year deal. Reports suggest PSV have paid the Lisbon club 1.6million euros for the pair.
A statement published on PSV's official website confirmed Schaars had successfully passed his medical with director Marcel Brands saying: "Stijn cannot wait to get going."
Schaars won the Eredivisie title with AZ Alkmaar in 2009 before leaving for Portugal two years ago. He has made 19 appearances for Holland.
Of Colombia Under-20 international Arias, who is equally comfortable on the right side of midfield or at full-back, Brands added: "We have tried to get him several times. I'm glad it's finally done."
Manager Philip Cocu has now added six new players to his squad following the recruitment of Florian Jozefzoon, Adam Maher, Karim Rekik and Jeffrey Bruma.