The 20-year-old has signed a long-term contract with the Anfield club after passing a medical, with the deal still subject to international clearance.
Alberto, capped at several levels of youth football by Spain, spent last season on loan with Barcelona B, where he scored 11 goals and collected 17 assists in 38 appearances.
"We are all delighted that Luis has decided to join Liverpool," Reds manager Brendan Rodgers said.
"He has the correct footballing profile and mentality to be a Liverpool player.
"I look forward to working with him to develop his talent and he joins a group of young exciting players at our club."
Alberto admitted he had been surprised when he became aware of Liverpool's interest, but had no doubts about accepting their offer.
"I feel like I'm in a bubble at the moment," he told Liverpoolfc.com. "For me Liverpool is a top team. I feel like I am dreaming and I want to enjoy it to the full.
"I feel very excited, I've never felt so excited before but I'm very happy. I think I am the happiest man on earth.
"I have come here to enjoy moments with the supporters - and I know the supporters will help me to never walk alone.
"I hope that between us all we can make this club one of the greatest in the world, once again."