Since joining the Catalan giants from Valencia in the 2015/2016 season, the Spaniard has made 50 appearances, scoring 15 goals in the process.
A statement on the club's website read:
'FC Barcelona and Borussia Dortmund have reached an agreement for the loanof the player Paco Alcácer for the 2018/19 season. The German club will pay FC Barcelona 2 million euros for the loan and will pay the player’s wages.
"Borussia Dortmund retains an option to buy the player for 23 million euros plus 5 million in variables and FC Barcelona has the right to match any future transfer offer of a third football club. Furthermore, FC Barcelona will retain 5% of any future profit from the sale of the player by Borussia Dortmund to a third party."