Manquillo becomes Liverpool's seventh summer signing.
Liverpool have completed the signing of Javier Manquillo from Atletico Madrid on a loan deal with an option to make the deal permanent, reportedly for a fee of £5 million.
The 20-year-old is a key signing for manager Brendan Rodgers as he wanted to strengthen the full-back areas ahead of the new campaign.
Manquillo can play on both flanks, having done so for Atletico Madrid, and is likely to provide competition for Glen Johnson and Jon Flanagan, but could fill in on the left side if required.
The Spaniard has made a total of 16 appearances for the current La Liga champions, and becomes the club's seventh signing of the summer, following Rickie Lambert, Adam Lallana, Emre Can, Lazar Markovic, Dejan Lovren and Divock Origi into the club.
He told the Liverpool website: "I'm very happy to have signed for a club like Liverpool. I think it's one of the biggest in Europe. Any footballer would want to come and play here.
"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.
"It's a team that nowadays, as I said before, any player would want to be a part of."
The Reds are expected to strengthen further between now and the start of September.