The fomer Sevilla forward has an impressive goalscoring record throughout his career, netting on 157 occasions in 376 appearances.
However, Negredo has struggled for form at Valencia in the last couple of seasons and scored just five times in 25 LaLiga appearances last season.
This will be Negredo's second spell in England having previously played for Manchester City. In his one season at the Etihad, the Spaniard scored 23 times in 49 appearances.
Negredo will join fellow countryman Victor Valdes who joined Boro on a free transfer last week.