The Spanish side announced this on their official website on Wednesday, having agreed terms with both the player and his former side Heerenveen.
The talks between both club began early last month and finally after getting a price which Heerenveen considered to be enough for arguably their most prized possession, a deal was struck.
It is believed that the striker will join for a fee of around 7.5 million euros which could move up to 10 million euros if certain performance related objectives are achieved.
Finnbogason has also passed a medical and has agreed to a contract which would keep him there until June 2018 with an official unveiling still yet to occur.