The 16-year-old attacking midfielder, who has played for Spain at youth level, is described by Monaco as "a great hope of his generation".
"Monaco are a great club that relies heavily on its training and gives young people the chance to express themselves at the highest level," Navarro said.
"My goal is to work hard to move forward and take new steps."
Monaco continues to attempt to restock their ranks with talented youth after seeing multiple stars depart the club over the past two seasons.
A Ligue 1 title and Champions League semi-final appearance in 2016-17 was dampened by the summer exit of teenage senasation Kylian Mbappe to domestic rivals Paris Saint-Germain, while Bernardo Silva and Benjamin Mendy both joined Manchester City.
However, in addition to Geubbels and Navarro, Monaco have also signed Troyes winger Samuel Grandsir on a five-year deal in preparation for the 2018-19 season.
Monaco finished second in Ligue 1 last season on 80 points, 13 behind winners PSG.