La Liga champion Barca has paid €18m up front with a further €12m potentially due in add-ons.
Junior Firpo, 22 years old, signed a contract with the Spanish club for 18 million euros plus another 12 million in variables, with a termination clause of 200 million euros, to play 5 seasons with Barcelona FC.
Despite being born in the Dominican Republic, he is a Spanish global with the U21 team.
Will Junior Firpo start for Barcelona?
"His electric pace and his tireless running see him quickly recover his defensive position".
The move marks a rapid rise for the left back, having only made his first team debut in February previous year against Deportivo la Coruña.
"In addition, Junior Firpo dominates in possession - a fundamental aspect of the Barca game".
They then kick-off their Liga campaign against Athletic Club six days later, though a row between the league and the Spanish Football Federation (RFEF) has thrown the kick-off time for that match into doubt.