The Spain Under-21 international arrives on a five-year deal for an initial fee of 15 million euros, with an option to extend for a further year.
The 21-year-old, who can operate both centrally and on the right side of defense, fills the vacancy left by Takehiro Tomiyasu, whose contract was mutually terminated earlier this month.
"We identified Cristhian as one of the strongest young defenders in European football, and he is an important signing for our future," said Arsenal sporting director Andrea Berta. "Cristhian is a versatile defender who will strengthen our squad."
Mosquera made 90 senior appearances for Valencia, becoming a regular starter last season and completing 37 of their 38 La Liga fixtures as the club finished 12th. He initially joined the Spanish side in 2016 and made his first-team debut in 2022, becoming the youngest centre-back in club history to feature in a league match.
"My career has moved faster than I expected and I've had to mature quickly," Mosquera said. "Coming here is a chance to keep learning and improving. I couldn't let it pass me by. I'm joining a historic, massive club."
Arsenal manager Mikel Arteta described the defender as a player with "enormous potential" and welcomed the addition as part of the club's longer-term strategy to blend experience with youth.
Mosquera is Arsenal's fifth confirmed signing of the summer, following Martin Zubimendi, Kepa Arrizabalaga, Christian Nørgaard, and Noni Madueke. Sporting CP striker Viktor Gyokeres is also reportedly close to a move to North London.
The Colombian-born defender will join the squad on its pre-season tour of Asia, having already received permission from Valencia to travel prior to the official announcement. Arsenal defeated AC Milan 1-0 in Singapore on Wednesday and are set to face Newcastle United at the same venue this weekend, before taking on Tottenham in Hong Kong.
With the 2025/26 Premier League season fast approaching, Mosquera's arrival provides added depth and a promising future option in Arsenal's back line as the club looks to build on last season's progress under Arteta.
Pau Victor's journey with Barcelona has drawn to a close, with the Catalan giants confirming on Friday that the 23-year-old striker from Sant Cugat has secured a permanent transfer to Portuguese side Braga.
Arsenal are on the verge of completing the signing of Viktor Gyokeres after reaching a total agreement with Sporting CP for a deal worth 63.8 million pounds.
Luke Shaw is reportedly open to considering offers this transfer window as doubts grow over his long-term future at Manchester United.
Chelsea have seen their initial bid for Ajax's highly rated young defender Jorrel Hato turned down, according to reports.
