The 24-year-old, who scored six times in 36 games for Almeria last season, completed his move to the Europa League winners on Tuesday.
"I've been living the dream for the last three years in La Liga," he told the club's official website.
"But this opportunity was one I couldn't pass up. I only have words of thanks for Almeria, who have helped me enormously in my career. But I am still young and I can improve – and I hope to do that here in Seville.
"I appreciate the opportunity that has been given to me and I want to repay the president and the coach, who believe in me. I feel great here already."
Vidal joined Almeria in 2011.