Lopez is 27-years-old and has 89 La Liga games to his name, with eight goals.
He is a central midfielder who has a contract at the club until 2018, and is a former Spain international, having represented his country at under-17 level.
Lopez played 28 times for Celta Vigo last season, scoring twice.
Sheffield Wednesday are planning a big promotion tilt next season under the new ownership of Thai businessman Dejphon Chansiri.
Stuart Gray last month was removed as manager, with Carlos Carvalhal appointed. He is to be unveiled at a press conference later today.
Sheffield Wednesday announced on their club website just last night that attacking midfielder Marco Matias had signed from Portuguese club Nacional.