The left-back has signed a five-year deal with the Spanish champions and will provide competition for Jordi Alba at Camp Nou.
The final fee for Junior could rise to €30m (£27.5m/$33.4m) with add-ons, while his release clause has been set at €200m.
Goalhas learned that those bonuses include €2.5m if Junior plays for the Spanish national side and a further €2.5m dependent on the trophies won during his time in Catalunya.
Born in the Dominican Republic, Junior has represented Spain at Under-21 level having impressed during five years at Betis.
The 22-year-old leaves the Andalusian outfit after making 43 appearances, scoring five goals and laying on seven assists in all competitions.
One of those goals came in a memorable 4-3 win over Barca at Camp Nou in November, with Junior having been linked with a move to the Blaugrana ever since.
“One of the young talents that shone during the past year in La Liga, Junior is a concern for rival defences,” Barcelona’s statement read.
“His electricity and his tireless running allow him to recover quickly the defensive position.