The 22-year-old has scored 11 league goals in three seasons with the Primera Division club but looks poised to leave Spain for Portugal permanently, with the loan arrangement allowing the Dragons to purchase Tello in two years' time.
A statement published on Porto's official website revealed Tello was preparing to fly to Holland and join his new team-mates for the final days of a pre-season training camp in Horst.
A European Under-21 champion with Spain in 2013, Tello slipped out of the first-team picture at the Nou Camp following the arrival of Neymar last summer, but did manage to score a hat-trick against Levante in the Copa del Rey.
He also scored against Celtic in the Champions League last season.
Tello becomes the second new arrival at the Estadio do Dragao in 24 hours after Dutch centre-back Bruno Martins Indi left Feyenoord to sign a four-year contract with Julen Lopetegui's side.