The deal is worth around STG15 million pounds ($A29 million) for the 26-year old, who was in the final year of his contract at United.
Wan-Bissaka began his career at Crystal Palace before joining United in 2019. He played 190 matches for the Manchester club and helped them win the FA Cup last season.
"I see a team that is in it together, that will push each other to win, and a group that has got each others' backs through thick and thin," Wan-Bissaka said in a statement.
"It was a no brainer for me to join West Ham - I'm excited and happy to be here."
West Ham, who finished ninth last season, kick off their Premier League campaign at home to Aston Villa on Saturday with new coach Julen Lopetegui at the helm.
Manchester City's Julian Alvarez celebrates a goal but he will be playing in Spain next season. (AP PHOTO)
Earlier, Atletico Madrid completed the signing of Julian Alvarez from Manchester City on a six-year contract.
The deal is understood to be worth over $124 million with a further $33 million in potential add-ons.
It represents a significant profit on a player signed from River Plate for $23 million and underlines City manager Pep Guardiola's willingness to let unhappy players leave.
He has won the Premier League twice and the Champions League, FA Cup, Super Cup and Club World Cup all once. He also won the 2022 World Cup with Argentina and the Copa America earlier this year.
Alvarez scored 36 goals In 103 appearances in all competitions for City, but despite being first choice for Argentina, he was behind Erling Haaland in the City pecking order.
The 24-year-old told City's website: "Today I say goodbye to this amazing club, with a lot of emotion. These have been two very special years. During this time, I have grown and learnt a lot - both as a player and as a person."
City director of football Txiki Begiristain said Alvarez's "attitude has been first-class since he first walked through the door here.
"That's why he has improved so much," Begiristain added. "The trophies he has won already tell their own story - he is a winner. I look forward to watching him develop further in the remainder of his career."
Brentford have completed the signing of forward Fabio Carvalho from Liverpool on a five-year contract.
The 21-year-old will initially cost the Bees $44 million with a further $10 million in add-ons, with Liverpool also entitled to a 17.5 per cent sell-on fee for a player they bought from Fulham for $10 million two years ago.
Brentford head coach Thomas Frank said: "He has great abilities on the ball. He can go past players, slide a pass, create chances and get on the end of chances. He's a type of player that we don't have many of in the squad so he will add that something extra for us."
AC Milan have completed the signing of defender Emerson Royal from Tottenham for a reported $25 million.
The 25-year-old Brazil international has agreed a four-year contract with the Serie A club, with the option of a further 12 months.
Emerson scored four goals in 101 appearances for Spurs after joining from Barcelona in 2021 for a reported fee of $50 million.
with Reuters
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