The 22-year-old, a product of Barcelona's academy who joined the club a decade ago, has made 75 senior appearances for the Catalan giants, contributing one goal and seven assists. The defensive midfielder has been strongly linked with a move away for some time as he searches for regular first-team football, with interest previously centred on Saudi Arabian clubs.
Sunderland's move represents a shift towards a Premier League destination for a player who has also attracted attention from Aston Villa, Newcastle United and Brighton & Hove Albion, all of whom continue to monitor the situation.
Barcelona's price tag for Casadó is understood to sit between €30million and €40million, with two years remaining on his contract giving the Spanish club some leverage in negotiations even as they push to move him on before the deadline.
Casadó has found opportunities increasingly hard to come by under Hansi Flick, with competition for places in Barcelona's midfield intense and the returning Frenkie de Jong further squeezing his path into the team.
His representatives are understood to be working on identifying the best possible destination, with the player himself said to prefer remaining in Europe at a club offering regular competitive football over a lucrative move to the Middle East.
The midfielder has won two senior caps for Spain, underlining the level at which he has operated despite his limited game time over the past year, and would represent a significant statement of intent from Sunderland as they look to strengthen further before the transfer window shuts in a week's time.
Sunderland's pursuit of Casadó forms part of a broader push to bolster their midfield options as head coach Régis Le Bris looks to build depth capable of sustaining a first season back in the Premier League.
Should a deal be completed, Casadó would arrive with a technical profile distinct from much of Sunderland's current midfield, offering composure in possession and defensive discipline honed within one of European football's most demanding academy environments, even if his recent lack of regular minutes means an adaptation period should be expected.