The 24-year-old winger has signed a long-term deal with
He becomes new manager Brendan Rodgers' third signing, following Fabio Borini and Joe Allen to Anfield.
Assaidi, who has represented Morocco 22 times, told the club's website: "I'm very happy to play for this big club. 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."
Assaidi scored 20 goals in 68 appearances for Heerenveen in the Dutch Eredivisie.
He becomes Brendan Rodgers' third signing as Reds boss following the arrivals of Fabio Borini and Joe Allen.
Rodgers said: "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.
"We're not expecting too much too early, he needs time to adapt but we're delighted he's chosen to come here over a number of other clubs.
"As I said, I'll look forward to helping his development and I hope he'll become a very good part of this club."
Rodgers also expects to complete a loan deal for Real Madrid and Turkey midfielder Nuri Sahin but it seems he must now wait until at least the new year to sign Barcelona winger Cristian Tello.
The Liverpool manager is still hoping to seal deals that will move the likes of Joe Cole and Andy Carroll on from Anfield - but only if the terms are attractive to the Reds.
Rodgers has also said he does not want to sell defender Daniel Agger, who has been linked with Manchester City.