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Liverpool did not confirm the length of the loan, but 20-year-old Manquillo is reported to have committed to a two-year stay at Anfield.
"I'm very happy to have
signed for a club like Liverpool," Manquillo told the Liverpool website.
"I think it's one of the biggest in Europe.
"Any footballer would want
to come and play here."
Manquillo began his career with Atletico's city rivals Real Madrid, but made the cross-town switch to the Vicente Calderon in 2007.
He made his first-team debut in 2011 and has represented Spain at every age-group from under-16 to under-20.
"When they tell you Liverpool want to sign you, I think like any player, you are going to be absolutely amazed and you don't think of anything else than going straight there," he added.
"The immediate objective is
to do the best I possibly can here.
"I've come here to try and secure a
place for myself in the side and, if possible, stay here for many more
years in the future."
Manquillo is expected to provide cover for first-choice right-back Glen Johnson, but he will also face competition from Jon Flanagan, who broke into the first team at left-back towards the end of last season.
He is the seventh player to join Liverpool during the close season after Rickie Lambert, Adam Lallana, Emre Can, Lazar Markovic, Dejan Lovren and Divock Origi, who has been loaned back to former club Lille.
Runners-up to Manchester City in 2013-14, Liverpool begin the new English Premiership season with a home game against Southampton on 17 August.
