Unal, who missed most of the first half of 2023/24 due to a cruciate ligament tear while at La Liga side Getafe, made 16 league appearances for Bournemouth since his arrival, scoring twice.
"His work rate and attitude have been exemplary following a long period of injury rehabilitation, and his desire to succeed in Premier League has been clear to see throughout," Bournemouth chief executive Neill Blake said in a statement.
"I enjoyed testing myself in the Premier League while regaining my fitness," said Unal, who assisted Antoine Semenyo's late winner in March as Bournemouth came back from 3-0 down to beat Luton Town 4-3.
"I am really excited for the years ahead," he added.