The 28-year-old France international joins the Irons on an initial season-long deal, with the option to make the transfer permanent in the summer of 2022.
He spent last season with Fulham, where he excelled and won the club's Player of the Year award.
“I'm happy," said Areola. “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."
Hammer manager David Moyes also told whufc.com: “Alphonse is an experienced and talented goalkeeper who has played at the highest level and performed very well in the Premier League last season," he said.
“As we prepare for a very busy schedule, he will provide fantastic competition for the goalkeepers already at the football club and we are going to need to use our squad to ensure compete to the best of our abilities across all competitions.
“We have been aware of his qualities for a long time and we have no doubt he will be another player and character who will enhance our squad. We welcome Alphonse to West Ham United and look forward to working with him."
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