Dortmund swoop for Haller to fill Haaland void
Haller, 28, had flown to Germany last month to undergo a medical with Dortmund following the sale of Haaland to Premier League champions Manchester City.
Ajax said they will receive a transfer fee of 31 million euros ($31.5 million) from Dortmund, an amount that could rise to 34.5 million.
"We're very pleased to have been able to sign Sebastien Haller," said Dortmund's new sporting director Sebastian Kehl.
"(He's) a seasoned centre-forward who has recently caused a sensation in the Champions League, but who also knows the Bundesliga in particular and has already demonstrated his class and high-quality finishing there."
Haller returns to Germany having spent two seasons with Eintracht Frankfurt from 2017 to 2019.
He had a spell at West Ham before joining Ajax in January 2021.
Haller struck 47 goals in 65 appearances for Ajax, notably scoring 11 times in last season's Champions League -- including in home and away wins over Dortmund in the group stage.
He helped the club to a league and cup double in his first season and then finished top scorer in the Eredivisie last term as Ajax retained the title.