West Ham had a busy end to the transfer window as they signed their ninth player of the summer - while two other players left the club for the season.
After completing the move, he told the club's website: 'I am really excited to be here as a West Ham player.
'It has always been a dream of mine to play in the Premier League, and to be doing it in London with a Club like West Ham is an incredible feeling.
'I can't wait to feel the support of the fans, who I have already heard so much about from conversations with people who know the Club, like my good friend Pablo Fornals.
West Ham have completed the loan signing of Carlos Soler from Paris Saint-Germain
'I am excited about meeting my new team-mates and giving my all for this Club in the best league in the world.'
Soler, who is capped 14 times by Spain, made his breakthrough with hometown club Valencia before later joining Paris Saint-Germain in the summer of 2012.
He made 63 appearances in the French capital, winning two Ligue 1 titles, a Coupe de France and a Trophee des Champions.
Meanwhile, James Ward-Prowse, Maxwel Cornet and Nayef Aguerd have left the club on loan.
English midfielder Ward-Prowse has made the move to Nottingham Forest for the 2024-25 season, while Cornet will play for Southampton.
The Hammers were one of the busier teams during the summer window, and both Ward-Prowse and Cornet have dropped down the pecking order under new manager Julen Lopetegui.
Aguerd meanwhile has joined LaLiga side Real Sociedad for the season.