The 17-year-old, who made his debut for Perth in March 2013 to become the club's youngest ever professional player, will be able to join the Serie A club next May for an undisclosed fee.
Perth Glory chief executive Jason Brewer told footballaustralia.com.au: "This is a great opportunity for Daniel and is a clear indication of the talent that he has.
"To have Daniel remain with Perth Glory for the upcoming 2014/15 Hyundai A-League season is also a great result for the club and I hope that our fans will get the opportunity to see him in plenty of action this year.
"This is also an important season for Daniel, who will be looking to establish himself in the Perth Glory team and will also want to impress ahead of the FIFA U20 World Cup in New Zealand next year.
"To start the season with this all now finalised is fantastic news."
Perth Glory chairman Tony Sage said: "This is great news for Daniel and also great news for Perth Glory fans. Daniel will continue to play for Perth Glory this coming season as he continues to develop into one of the best footballing talents this country has ever produced."
De Silva will join up with Glory for pre-season training on June 18.