Costa's international future has been the subject of much speculation, with both Brazil and Spain keen to have the striker in their squad.
The Brazilian-born forward has dual nationality after residing in Spain for more than five years, and was said to have stated his wish to change allegiance to the world and European champions earlier this month.
However, the 25-year-old's prospective switch to Spain had been put on hold after FIFA requested confirmation that Costa - who has won two caps for Brazil in his career - has never represented the South American nation in a competitive fixture.
But Costa now appears free to represent Spain after he signed a letter to the Brazilian Football Confederation (CBF) declaring his intention to switch allegiance on Tuesday.
A statement on the Royal Spanish Football Federation (RFEF) website read: "Brazilian-Hispanic player Diego Da Silva Costa "Diego Costa" has signed a latter manifesting his intention to play for the Spanish football team.
"In the letter addressed to Mr Julio Cesar Avelleda, secretary general of the CBF, the striker communicates his desire to be at the disposal of Spanish national coach Vicente del Bosque as of this moment.
"Diego Costa can play with Spain, in the next match of the month of November, if selected by the national coach."
Costa's debut for Spain could come next month, when they face South Africa in a friendly on November 19.
However, the RFEF are reportedly expecting the CBF to appeal to FIFA over Costa's eligibility to play for Spain, on the basis that he has already represented Brazil twice in March against Italy and Russia.
Costa has scored 13 goals for Atletico so far this season, already topping his previous best return from a single campaign.
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