Aguilar, a holding midfielder with 41 caps to his name, has joined from Spanish second division club Hercules for an undisclosed fee and will replace Etienne Capoue, who left for Tottenham Hotspur last week.
The 28-year-old Aguilar is the second signing in less than 24 hours for Toulouse after they brought in midfielder Clement Chantome on a season-long loan from Paris St Germain.
Chantome was not a first choice at champions PSG and asked to leave the club to entertain his hopes of being called up again by France ahead of the World Cup after winning his first and only cap last year in a friendly against Japan.
The 26-year-old Frenchman, who played 28 league games with PSG last season, has made the squad for Friday's trip to big-spending Monaco.
Aguilar has joined too late and will only be available for the following weekend's fourth round of matches.