Arsenal sign Valencia defender Mosquera
The Spain under-21 international joins for an initial fee of around £13 million, with performance-related add-ons, and is understood to have agreed a five-year deal with an option of a further year.
The 21-year-old, Arsenal's fifth signing in recent weeks, is set to provide cover for William Saliba and Gabriel at the heart of the Gunners' defence, but can operate anywhere along the back line.
Mosquera, who played 41 times for Valencia last season, has joined Arsenal for their tour of Singapore and Hong Kong.
"It means a lot to me," he told the club website. "There's been a lot of hard work behind all of this.
"It's an opportunity that came up and I couldn't let it pass me by. I'm joining a historic, massive club. When you arrive here with your family, you really feel the greatness of this place."
Manager Mikel Arteta described Mosquera as "an intelligent player with good pace, who can play centrally and on both sides".
Mosquera follows Spain midfielder Martin Zubimendi, goalkeeper Kepa Arrizabalaga, Christian Norgaard and Noni Madueke in moving to the Emirates.
Gabriel and Myles Lewis-Skelly have also put pen to paper on new contracts in the off-season, with a long-term extension for teenager Ethan Nwaneri believed to be all but completed.
Arsenal remain hopeful of signing striker Viktor Gyokeres from Sporting Lisbon, although it is not believed a deal, which could be worth as much as £65 million, is imminent.
The Gunners, who have finished as Premier League runners-up three times in a row, launch their 2025/26 campaign at Manchester United on August 17.