Ward-Prowse was part of the Saints side relegated to the Championship last season.
The 28-year-old has secured a swift return to the top flight on a four-year contract that has cost West Ham £30m.
Ward-Prowse came through Southampton's youth system and departs the Saint Mary's Stadium with 410 appearances, 55 goals and 54 assists to his name.
The England international is West Ham's second summer signing following the arrival of Edson Alvarez from Ajax Amsterdam last week.
"I'm buzzing to be here at West Ham United. It's great to be here and I'm looking forward to getting going," Ward-Prowse told West Ham's media channels.
"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."
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."
Only David Beckham(18) has scored more direct free-kick goals in Premier League history than Ward-Prowse(17).
