Feghouli joined West Ham on a free transfer from Valencia on Tuesday.
The Hammers sealed the signing of winger Feghouli on a free transfer from Valencia on Tuesday to add to the arrival of defensive midfielder Havard Nordtveit earlier this summer.
The 26-year-old has played a starring role for the Spanish side over the last few years, and it is thought that West Ham beat off competition from several other clubs to secure his signature.
Feghouli is also an established Algerian international having played 40 times for his country and featuring at the 2014 World Cup, and he is an international teammate of Leicester City star Mahrez.
Hammers co-chairman David Sullivan believes the club have got another top-class player on their hands, and suggested that Feghouli is considered as big a star as Mahrez in his home country.
Sullivan told the club website: "It’s a deal myself and my team have been working on for many weeks. I had a lengthy meeting with his agent ten days ago.
"He's an outstanding player and as big a star as Mahrez in his own country. If he adapts to the Premier League we have signed another very special player with experience all over Europe via the cup competitions for his club."
Feghouli plays generally as a winger on the right but has been known to feature from the left or centrally on occasions in his five years as a regular for Valencia.
He will add to an ever-improving attacking midfield line-up which already boasts talent including Dimitri Payet, Manuel Lanzini, and Michail Antonio as West Ham look to kick on from last season's seventh-placed finish to launch a genuine assault on the top four next term.