McGree's loss will hit Adelaide hardest, with the 21-year-old moving after a stunning domestic season in which he claimed the Reds' club champion award.
"My goal has always been to push myself and my football, and to have the chance to head overseas and to be doing that is very exciting," he said in a statement on Tuesday.
Adelaide's director of football Bruce Djite said it was inevitable McGree, who returned to the A-League after a stint with Belgium's Club Brugge, would return to Europe.
"Riley had an outstanding season last year and has been rightfully rewarded," Djite said.
"It is always difficult when one of the club's best talents departs and Riley will be very difficult to replace.
"However when players have hunger and desire to play at the highest level, and they get that opportunity, we as a club will always look to support them."