The 26-year-old made 109 appearances for the La Liga club since joining from Villareal in 2020, scoring 26 goals in all competitions.
Unal, who has 32 caps for Turkey, has only played in three La Liga games this season and started just one match.
He was bought by Manchester City in 2015 when he 16, but made no competitive appearances for the club, instead spending time on loan with Genk, NAC Breda and Twente.
Neill Blake, AFC Bournemouth’s chief executive, said: “We’re really pleased to welcome Enes to the club and believe he’ll add strength to our front line.
“He’s proven that he can score goals in multiple different countries, and we believe he’s got the skillset to succeed in the Premier League.”