Dortmund will pay €2 million up front for the loan and can commit him to a four-year deal if they hand over a further €23m (£20.9m/$27m) at the end of the season, though the Catalan side can reject that offer. With a further €5m in variables, the 24-year-old could end up costing €30m.
Alcacer had his medical at the German club on Tuesday after Liverpool pulled the plug on a prospective move for Divock Origi, and the Spain international is excited to begin his Bundesliga adventure.
“I’m really looking forward to playing in the Bundesliga soon," Alcacer said.
"It’s one of the best leagues in the world and BVB, with their unbelievable fans and their famous South Stand, probably represent something magical for every football player."
Alcacer was unable to secure a regular spot in the Barca first-team since joining from Valencia in 2016. The Spain international played just 17 times in La Liga last term and did not feature for Ernesto Valverde's team in the early stages of this season.
However, he had been a success at Valencia, where he scored 43 goals in 124 games over a six-year spell.
And Dortmund sporting director Michael Zorc is confident Alcacer can find his form in Germany and prove a good addition to Lucien Favre's team.
"We’re absolutely delighted that Paco Alcácer has chosen to join Borussia Dortmund and we’re firmly convinced this type of player will be a good fit for our style of play," he said.
"Paco proved his qualities during his younger years at Valencia, scoring lots of goals, and acquired special status there at just 22 years of age when he was handed the captain’s armband. In Barcelona, he worked alongside some of the best players in the world on a daily basis.
"The fact that he’s now taken the decision to make a fresh start after two years is attributable to the extremely tough competition for attacking places at the club.”