The move brings an end, for now, to the 22-year-old's first-team career at Camp Nou, where he had made 75 appearances since breaking through under Hansi Flick and won two La Liga titles, a Copa del Rey and two Spanish Super Cups along the way.
Barcelona will cover the majority of his wages during the loan, which runs until the end of June 2027, as Deportivo continue a striking recruitment push that has also brought in Pierre-Emerick Aubameyang, Adama Traoré, José María Giménez and Leo Román since the club's return to the top flight.
Speaking to reporters at El Prat airport before departing for Galicia, Casadó was measured when asked whether he was frustrated with how his Barcelona exit had been handled.
"We've already spoken with the club about how they've managed things, how they've done it, and that's it. We all have things to improve, we've all done things well and things to improve, and that's it," he said.
The midfielder acknowledged the difficulty of leaving the club where he came through the academy.
"Playing a year at Barça is always complicated," he said, before turning to his enthusiasm for the move itself, saying Deportivo had offered him "a really nice project" at Riazor and that he was excited about what lies ahead.
Asked whether he was leaving on bad terms, Casadó was direct: "I'm not leaving upset. I've spoken with the club and we all have things to improve."
Casadó had attracted interest from clubs in England, Turkey and Saudi Arabia during the window, along with RB Leipzig, but all of those options were ultimately discarded in favour of the move to Deportivo.
Footage released by his new club showed the midfielder's first hours in Galicia, touring the training complex alongside sporting director Fernando Soriano and fellow arrival Giménez. Casadó was visibly taken aback by the facilities, particularly the dressing room.
"This is better than at Barça, and everything," he said, prompting a surprised Soriano to ask if he meant it.
"Really," Casadó replied.