The fee is €19.5million fixed, with a further €4.5million in performance-related bonuses and a ten per cent sell-on clause included in favour of Salzburg. Dortmund announced the deal on Tuesday afternoon.
Gadou, who stands 195cm tall and is right-footed, joins as the direct replacement for veteran defender Niklas Süle, 30, who announced his retirement from professional football earlier this month after the club confirmed his expiring contract would not be renewed. Borussia Dortmund are also without veteran midfielder Emre Can, who has been sidelined since rupturing his anterior cruciate ligament in February.
Gadou came through Paris Saint-Germain's academy between 2020 and 2024 before Salzburg signed him for €10million. He made 58 appearances for the Austrian club across two seasons, including all three of their group games at the FIFA Club World Cup in Philadelphia last summer — one of which was against Real Madrid — and also featured in the UEFA Europa League.
Chief executive Lars Ricken spoke warmly of the deal.
"We've known about Joane for a long time and were watching him at PSG. Joane will bolster our ranks and take on an important role with us from the start of the new season. We're convinced of his qualities and see enormous potential for development."
Book, who only took over as sporting director from Sebastian Kehl this month, described what the club are getting.
"Joane is a modern and physically strong centre-back. He has good build-up play, is extremely fast and still has room to develop. Joane is the optimal addition for our defence."
Gadou said he had been looking forward to the moment.
"I am so happy to be part of the BVB family in the future. I can hardly wait to wear the black and yellow shirt for the first time. Together with my teammates, the whole club and our incredible fans, I want to be successful here in the coming years."
Dortmund finished second in the Bundesliga this season and have qualified for the Champions League. Gadou is the first piece of a summer rebuild that will also need to address the midfield depth lost with Süle's departure from the centre of defence.