Cadena SER first reported the loan, which comes with a purchase option that converts into an obligation under certain conditions, according to Fabrizio Romano's later update confirming the fixed €3.5m fee for a permanent deal next summer.
The 19-year-old, a Germany youth international, is initially expected to join Barcelona's reserve side rather than Hansi Flick's first-team squad.
Ćatović made 29 appearances for Stuttgart's third-tier side last season but never broke through to senior first-team football at the Bundesliga club, prompting the move to Spain in search of regular game time.
The defensive midfielder's departure continues VfB Stuttgart's pattern of allowing academy talent to leave for senior football elsewhere when a clear pathway isn't immediately available at Bundesliga level.
For Barcelona, the deal reflects the club's ongoing strategy of scouting and developing young talent from across Europe's top leagues, even for positions in the second team, betting on players with sufficient potential to eventually break into contention for Flick's senior squad.
Should Ćatović impress during his time with Barcelona Atlètic, the purchase option gives the Catalan club a relatively low-cost route to securing his services permanently without committing significant resources upfront.
The move is a modest one financially compared to Barcelona's headline pursuits elsewhere this summer, but fits a familiar pattern for the club's recruitment department: identifying promising teenagers who have struggled to break through at their current clubs and offering them a fresh environment in which to develop.
Ćatović's versatility in central midfield, combined with his existing experience of German youth international football, made him a low-risk addition even without any guarantee of a rapid path to the first team.