The Ecuador international will will leave Spain, having spent a total of three years with the Liga club for an undisclosed fee
The Ecuadorian winger moved to El Madrigal in 2009, but had failed to cement a regular starting spot with the Yellow Submarine.
He was subsequently sent on loan to Levante and Real Betis, where he managed to secure a regular starting spot, appearing 34 times this season, helping the club retain their Liga status with a 13th place finish.
However, the 22-year-old will now leave Spain has now agreed a three-year deal with La Monarquia for an undisclosed fee, and will join up with the squad in the coming days, and will undergo physical examinations.
Montero has represented his country on 19 occasions, scoring twice.