Adrian, who can play as forward or out wide, moves to Portugal after struggling to keep a regular place in the Atletico side under Diego Simeone.
He did however, play a key role in Atletico's Champions League quarter-final victory over Barcelona after Diego Costa was ruled out with an injury and he then went on to score against Chelsea in the semis.
Porto have slapped a €60m buy-out clause into Adrian's contract.
Adrian, who has two caps for Spain, joined Atletico from Deportivo La Coruna in 2011.
In three seasons with the club, he scored 26 goals in 142 appearances and helped them win their first La Liga title in 18 years last season and reach the final of the Champions League.
He also won the 2012 Europa League and the King's Cup and European Super Cup in 2013.
- Sports & Recreation
- Champions League
- Diego Costa
- Deportivo La Coruna