The Gunners forward has sealed a move away from the north London club having found first team opportunities difficult to come by during his debut season in the Premier League
The Spanish club, who have recently returned to La Liga for the first time in four years, revealed the news on their official website, with the South Korean having found first team opportunities difficult to come by in London.
The 27-year-old forward has made only six appearances for the Gunners since signing from Monaco in August of last year, scoring his solitary goal for the club against Bolton in the fourth round of last season's League Cup.
Recently Park suffered the indignity of losing his No.9 jersey to new signing Lukas Podolski, and was relegated to the No.30 shirt.
However, the striker had a successful summer as he was selected for South Korea's Olympic squad, scoring two goals - including the crucial opening goal in their 2-0 bronze medal victory over Brazil.