The 21-year-old becomes Mikel Arteta’s fifth addition of the summer, adding depth to the Gunners’ defensive ranks ahead of the new season.
He follows compatriots Martín Zubimendi and Kepa Arrizabalaga to north London, alongside Noni Madueke and Christian Norgaard.
"We're delighted to welcome Cristhian to Arsenal," said Arteta. "As a 21-year-old defender, Cristhian has already performed consistently well with significant experience in La Liga.
"He is an intelligent player with good pace, who can play centrally and on both sides."
Arteta added: “He is a young player with exciting talent and enormous potential, who will fit well into our squad as we continue to prepare for next season.
"Cristhian adds good quality to our squad, and we look forward to working with him and making him feel at home with us."
Mosquera, a product of Valencia’s academy, made his senior debut in 2021 and established himself as a regular in the first team by 2023/24.
Primarily deployed at centre-half, he's also comfortable operating on the right side of a back four. He has been praised for his composure on the ball and positional awareness.
With his Valencia contract set to expire next summer, and talks over an extension reportedly stalling, Arsenal moved swiftly to secure the highly-rated defender at a cut-price fee.
Los Murciélagos are understood to have made efforts to keep Mosquera, but the Premier League's lure and a chance to work under Arteta proved decisive.
Mosquera is likely to link up with his new teammates once they return from their pre-season tour in Asia.
