The 35-year-old Fluminense player was tested after the club's match with Boavista last month and his sample was found to contain traces of furosemide, a diuretic which is banned as it can be used to mask other substances.
Fluminense issued a short statement on their website to acknowledge the results, but offering no comment until the players 'B' sample is revealed on Friday.
According to reports in Brazil, Deco plans to fight any suspension, claiming the result is most likely a result of contamination to a vitamin supplement he has used for several years.
Felleger Ahaz, a spokesman for the player, said in O Globo: "The first concern for Deco is defending his image and not with any punishment or suspension.
"He has for a long time taken a vitamin supplement and certainly there has been some contamination."