James Ward-Prowse has completed his £30m move from Southampton to West Ham.
The midfielder brings to an end a 20-year spell on the south coast, having joined Southampton from the age of eight.
He joins the Hammers on a four-year contract until June 2027.
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'West Ham have real momentum'
"I'm buzzing to be here at West Ham United. It's great to be here and I'm looking forward to getting going," said Ward-Prowse, who is West Ham's second summer signing following Mexico midfielder Edson Alvarez.
"From the outside looking in, this is a Club that has been on the rise for a number of years now and coming off the success in the UEFA Europa Conference League last year, you can feel there is real momentum around this Club. It's great to be a part of it and I can't wait to play my role in the games to come.
"When I think about my game, it is based on hard work, graft, and giving 100 per cent. West Ham United has always been a club that has epitomises that. You can feel that from the fans and you can see it in the players who are here and the lads that have come through the Academy too.
"I feel as though I will fit straight in and I can promise I will give my all for this football club in my time here."
Moyes: Ward-Prowse's leadership qualities are a huge plus
Hammers boss David Moyes added: "I'm delighted to bring a player of James' quality and experience to West Ham United.
"He's performed to a very high level in the Premier League over the last decade, constantly producing good numbers season after season.
"His leadership qualities are obviously another huge plus for us - he'll be a big voice for us, someone who drives the team on from the middle of the pitch.
"We're excited to get him up and running, for what will be another busy season on both the domestic and European front."
Ward-Prowse played in Saints' opening Championship game, assisting Che Adams' winning goal against Sheffield Wednesday, but did not feature in the squad for Saturday's 4-4 draw with Norwich ahead of his West Ham move.
The Hammers had reportedly walked away from the deal in early August after making a 'take it or leave it' offer, but returned to negotiations before eventually landing the midfielder.
Ward-Prowse made 410 appearances for Southampton's first-team and scored 11 goals in all competitions in each of he last two seasons for the Saints - but could not keep them in the Premier League last term.
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