Boro will have the option to sign the 30-year-old on a permanent deal should the Negredo's spell at the Riverside be a success.
Negredo has pent two seasons at parent club Valencia, scoring 18 goals in 76 games.
This will be the Spaniard's second spell in England, having spent one season at Manchester City in the 2013-14 season where he scored 23 goals in 49 games.