Liverpool have completed a £55m deal to sign midfielder Alexis Mac Allister from Brighton on a five-year contract.
The Argentina playmaker was a central figure as Brighton finished sixth in the Premier League last term to qualify for Europe for the first time.
The 24-year-old made 112 appearances in total for the Seagulls, scoring 20 goals.
"It feels amazing. It's a dream come true and I can't wait to get started," said Mac Allister.
"I wanted to be in [from] the first day of pre-season, so it's good that everything is done. I'm looking forward to meeting my teammates.
"It was a fantastic year for me - World Cup, what we achieved with Brighton - but now it's time to think about Liverpool and try to be a better player and a better human being every day."
Reds manager Jurgen Klopp is looking to reinforce his midfield with James Milner, Naby Keita and Alex Oxlade-Chamberlain all leaving Anfield this summer.
They were priced out of a move for Borussia Dortmund's England teenager Jude Bellingham, who looks set to join Real Madrid after the clubs agreed a 103m euros (£88.5m) fee.
Mac Allister joined Brighton from Argentinos Juniors in January 2019 and immediately went back to the Buenos Aires club on loan.
He has played 16 times for Argentina and emerged as a key player during their third World Cup triumph in December.
Having sat out their opening defeat by Saudi Arabia, he then started the next six games for Lionel Scaloni's team and starred in the final against France.
"Since I won the World Cup, I said that I want to win more trophies and I think that this club will help me to do that," said Mac Allister.
"That's the aim and when you are in a big club like this one you have to win trophies. I can see how big this club is - the players we have, the staff, everyone. I'm really pleased and looking forward to playing for this club."