The Uruguayan has signed a four-year contract at the club after failing to make an impact in Serie B, where he spent three of his four years out on loan.
Stuani is the seventh signing of the season for the La Liga side and he spoke of his delight upon sealing the move.
“I am very happy to play at Espanyol because it is a team that has everything to grow," he said.
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Deportivo’s president Augusto Cesar Lendoiri told press last week that Stuani was a primary target for the newly promoted side.
But after his agent confirmed that the young striker had completed a medical at the Riazor Stuani opted to turn down their offer when Espanyol made a late approach for his services.