The Merseyside club have confirmed the capture of the 24-year-old after the Premier League side agreed a deal with the Dutch outfit yesterday.
"I'm very happy to play for this big club," Assaidi told the club's official website. "The first time I spoke to the coach I had a good feeling. I had offers from other big clubs but I've been waiting for this moment.
"The coach gave me a good feeling. It will be a dream to play with these big players and I hope training will help me to be a good player here. When I watch Liverpool it's always a good atmosphere, the fans are fantastic and I'm happy to be here."
Liverpool boss Brendan Rodgers added: "We're delighted with the signing, he's a young player who has done very well, in particular in the last couple of seasons, in Holland.
"He fits our style perfectly, he's an exciting player who is very good in one versus one situations and I think he's a player that will excite the crowd."
Assaidi, who scored 20 goals in 68 appearances in the Eredivisie, has become the Reds' third signing of the summer following the acquisitions of Fabio Borini and Joe Allen.