The 24-year-old has signed a five-year deal to join the Saints from Feyenoord for a reported fee of £8.4million.
“It feels good to be joining Southampton. It's a nice club, and I'm happy to be here," Clasie told the club's official website.
“It's a big club playing in what I think is the biggest and most important league.
“Ronald being here is a big thing for me and that was important in me coming here, but even if he wasn't here, I'd have come to this club.
“I played every game for him. He's a good trainer and a good man so I like him, but I came here not only for him. I came here for the club.
“I've watched many games of Southampton, and it's felt good when I've watched these games.
“I hope that I have many good moments and good years here."
CONFIRMED: #SaintsFC complete the signing of @Clasie_Official on a five-year deal from @Feyenoord. https://t.co/ewewNAonQS
— Southampton FC (@SouthamptonFC) July 15, 2015
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