Pressure has mounted on Catalan president Carles Puigdemont to back down after hundreds of thousands of protesters last week rallied to defend national unity.
"If there were a declaration of independence it would be unilateral and it wouldn't be recognised," Nathalie Loiseau said on CNews digital news channel.
"This crisis needs to be resolved through dialogue at all levels of Spanish politics," she urged.
The minister also reiterated Brussels' warning that an independent Catalonia would "automatically" be out of the EU and have to reapply to join.
She said Catalonia, which borders France, already has "considerable autonomy".