The 22-year-old has penned a "long-term deal" at Anfield after successfully passing a medical and agreeing personal terms.
Moreno, who has won three caps for Spain, was part of the Sevilla side that won the Europa League last season.
The youngster is not eligible to play for the Reds in their Premier League opener against Southampton on Sunday.
"In my view, Liverpool are the best club in England and I couldn't waste an opportunity to come and play for them," Moreno told the club's official website.
"I'm very happy to be joining a club as big as Liverpool. I hope to give my best and do all I can to help."
Moreno has been given the number 18 shirt for Liverpool.