It is a return to German football for the 28-year-old striker, who played for Eintracht Frankfurt from 2017-2019.
He has signed a four-year deal with Dortmund, last season’s runners-up in the Bundesliga, for a free that could increase to 34.5m euros.
“I can’t wait to feel the support of more than 80,000 spectators in the future,” Haller told the club website.
“We’ll hopefully achieve great things together in the next few years and be able to celebrate together too.”
In his 18 months at Ajax, former West Ham United man Haller scored 47 goals in 66 games, with 11 of these coming in last season’s Champions League when he broke several scoring records.
He reached that tally in just seven games, during a run in which he became only the second player after Cristiano Ronaldo to score in every group match.
“We’re very pleased to have been able to sign Sebastien – a seasoned centre-forward who has recently caused a sensation in the Champions League,” said Dortmund’s new sporting director Sebastian Kehl.
“[He] also knows the Bundesliga in particular and has already demonstrated his class and high-quality finishing there.”
During his previous spell with Frankfurt, Haller netted 24 goals in 60 appearances.
“The overall package is very promising,” continued Kehl. “Sebastien possesses a strong physical presence and he is physically very resilient.
“With his experience, he can also offer support and stability to our young attacking players. He made it clear that he has big ambitions with BVB.”
Haller, who will wear the number nine shirt, will help fill the void left following Erling Braut Haaland’s departure to Manchester City.
“Thank you and good luck,” Ajax said on social media to the man who finished top scorer in the Eredivisie last season with 21 goals.
Haller represented France at youth level before switching allegiance to Ivory Coast, and has also previously played for Auxerre and Utrecht.
