This transfer marks the conclusion of Ward-Prowse's two-decade association with Southampton, a club he first joined at the tender age of eight. Over the years, he progressed through the ranks and eventually ascended to the role of captain for the senior team.
Displaying steadfast loyalty, Ward-Prowse accumulated a remarkable tally of 410 appearances over his 12-year stint with the club. Notably, he participated in almost every Premier League match, only missing two games across the past four seasons, until Southampton experienced relegation to the Championship in the previous term.
Upon his move to West Ham, Ward-Prowse will don the number seven jersey. He recently took part in Southampton's season opener against Sheffield Wednesday earlier this month. Now, with his transfer complete, he is eligible to make his debut for the Hammers in the upcoming London derby against Chelsea this Sunday.
"I'm buzzing to be here at West Ham United. It's great to be here and I'm looking forward to getting going," the England international told the club's website.
"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%. 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."
Manager 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."