The France-born Ivory Coast international has signed a four-year contract at the Signal Iduna Park and comes in as the direct replacement for Erling Haaland, who joined Manchester City earlier this summer.
The move marks Haller's second stint in Germany, having impressed with Eintracht Frankfurt between 2017 and 2019 before moving to West Ham United, where his goals dried up despite the Hammers forking out £45m for his services.
Haller only scored 10 goals in 48 Premier League games.
A move to Ajax in January 2021 reinvigorated Haller's career as he struck 47 goals and provided 16 assists in 65 appearances across all competitions for the Dutch outfit.
Haller won two Eredivisie titles and two Dutch cups during his time with Ajax.
The 28-year-old was the third top scorer in the Champions League last season with 11 goals, behind only Robert Lewandowski(13) and Karim Benzema(15).
"We are very pleased that we were able to sign Sebastien Haller, a seasoned center-forward who recently caused a sensation in the Champions League, but who also knows the Bundesliga in particular and has already proven his class and high finishing quality there," Dortmund's new sports director Sebastian Kehl said.
"The overall package is very promising. Sebastien has a high level of physical presence and is also very resilient physically. With his experience, he can give our young players support and stability on the offensive. In the talks with us, he made it clear that he would like to achieve something with BVB."
Speaking of his move, Haller said: “It's a great stadium, but the greatest stadium is nothing without its fans.
“I can hardly wait to be able to feel the support of more than 80,000 spectators as part of the BVB family.
“Hopefully, we will achieve great things together in the coming years and then be able to celebrate together.”
