After an impressive start to the season in which he has scored 10 goals in nine La Liga games, Costa has been at the centre of a tug-of-war between Spain, where he has played for seven years, and his native Brazil.
With both countries showing great desire to claim Costa, the Atletico striker has said that has has now made up his mind.
"The decision has been made. The people that need to know are already aware of my decision," he told Jornal do Estado.
"It's very clear to me what I want to do and now I have to wait until the moment comes.
"If I could have said it before, I would have said it a long time ago."
Currently leading the scoring charts in La Liga ahead of Cristiano Ronaldo and Lionel Messi, Costa's international future looks likely to be settled in time for next year's World Cup in Brazil.