However, the former Chelsea star will pay a hefty fine to avoid serving the time.
The 31-year-old, who also played for Chelsea between 2014-2017, appeared in court in Madrid on Thursday morning dressed in a white mask.
According to a court filing published last month, Costa failed to declare payments of around €5.6m (£5m) following his move from the Blues to Atletico in 2014.
El Mundo reports he has been found guilty and handed a six month jail sentence.
However, in compliance with the law in Spain he has agreed to pay a fine of €507,208 (£453,000) as a punishment for defrauding the state as well as an additional €36,500 (£32,600) to avoid jail time.