"I'm excited about playing in The Premier League," Donk told the club's official website. "It's a big competition."
Expanding on his decision to move on, Donk said: "I've known Gianni for around five years - he's like my little brother.
"Gianni told me West Brom was a family club and that the trainer likes young players.
"That was one of the main reasons I came here.
"I'll be staying with Gianni for a couple of months while I settle in.
"It will make things a lot easier for me because of the support he can give me."