According to media reports, the transfer takes the Anfield club's spending in the window to more than 100 million pounds.
Liverpool sold controversial striker Luis Suarez to Barcelona for an estimated 75 million pounds during the close season.
"Alberto Moreno will wear the number 18 jersey at Anfield," the five-times European champions said on their website (www.liverpoolfc.com).
The 22-year-old graduated through the ranks at Sevilla and made his debut for Spain in October.
"In my view Liverpool are the best club in England and I couldn't waste an opportunity to come and play for them," said Moreno.
"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's move was not registered in time to be included in the team for their Premier League opener against Southampton at Anfield on Sunday.
He was named in Spain's provisional 30-man list for the 2014 World Cup in Brazil but did not make the final squad.
(Writing by Steve Tongue, editing by Tim Collings)
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