The right-back, who has signed a four-year deal with the Hammers, has become manager Manuel Pellegrini's first signing for the London club.
Fredericks will join up with the West Ham squad on July 1.
"It's amazing for this deal to finally happen," the defender told the official Hammers website. "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 was named in the PFA Championship Team of the Year last season.