Areola has joined on a season long deal from Paris Saint-Germain after undergoing a medical.
West Ham have an option to make Areola’s move a permanent one next summer.
Areola spent last season on loan at Fulham and impressed despite their relegation to the Championship, winning the club’s player of the year award.
But the 28 year-old has been pushed further down the pecking order at PSG by the arrival of Italy stopper Gianluigi Donnarumma.
West Ham identified Areola as their No.1 target to bolster their goalkeeping department this summer.
With first choice Lukasz Fabianski, 36, Areola will provide competition and potentially succeed him long-term.
West Ham also have Darren Randolph and David Martin as senior goalkeepers while they have allowed youngster Nathan Trott to continue his development with a season-long loan at French club Nancy.
Areola said: ‘For me it was the best opportunity to sign for this Club, to sign for West Ham and to try my best to help the team.
‘London is a great place to live and to work as well, and obviously the Premier League is one of the best leagues in the world so I really enjoyed it last season. It was tough for me and for the team, but I enjoyed playing in the Premier League. We were without the fans, but this year we will have the fans back so I can’t wait to see the atmosphere.
‘West Ham did a big season last year. Obviously we have the Europa League and the Premier League and the two cups, so it will be a tough season and a long season but I think we’re going to enjoy it all together.
‘We have really good squad. I watched games last season so I know the potential and the talent in this squad, so I can’t wait to start.’