The 25-year-old was in electric form at Craven Cottage last season, and helped the Cottagers seal promotion back to the riches of the Premier League with a 1-0 win over Aston Villa in the Championship play-off final at Wembley.
The former Tottenham full-back was out of contract though in West London, with Fredericks now choosing to continue his career at the London Stadium. He will team up with new Hammers boss Manuel Pellegrini there, where he will fight it out for a first-team place against the experienced Pablo Zabaleta.
Speaking to the club's official website, Fredricks - who becomes Manuel Pellegrini's first signing - said: "It’s amazing for this deal to finally happen.
"I know we’ve been speaking about it for a while and trying to get things in place and finally, we’ve got it over the line. I’m just buzzing to be here.
"West Ham United is one of the biggest clubs in the Premier League. Everyone knows the history of West Ham and there’s a lot of factors that brought me to the club.
"Obviously, it’s an exciting time for West Ham. The club have just brought in a new manager in Manuel Pellegrini and everyone knows his CV and what he has done in England before and in other leagues as well. I’m sure I can learn a lot from him."
Fredericks made 114 appearances in all competition for Fulham during a three year stay, establishing himself this season as one of the most feared right-back's in the Championship, in part due to his blistering pace and attacking intent.
His marauding forward runs are likely to blend in well with Pellegrini's attacking style of play, with the Chilean expected to overhaul the majority of the West Ham playing squad this summer in order to develop a more positive philosophy.
West Ham will now look to press on and make use of the reported £75m transfer kitty available to Pellegrini, with Lazio winger Felipe Anderson and Leicester striker Islam Slimani among a number of reported transfer targets.