The club confirmed the news on Tuesday via Twitter:
Bosz leaves Ajax after just one year at the club, having guided them to the UEFA Europa League final and to second place in the Eredivisie, one point behind champions Feyenoord.
Bosz's Ajax side were an exciting team brimming with promising young stars, many of them teenagers, including the likes of Kasper Dolberg, Matthijs de Ligt and Justin Kluivert to name but a few.
While they fell short both domestically and in Europe, his ability to get the best out of young talent will serve him well with BVB, who have a similarly outstanding crop in their ranks, per Squawka Football:
The 53-year-old has replaced Thomas Tuchel in the hotseat after he was sacked despite delivering the DFB-Pokal trophy, likely as a result of his strained relationship with the powers that be at the club.
Prior to joining Ajax, Bosz had spells with Maccabi Tel Aviv, Vitesse Arnhem and Heracles, among others. He is yet to win a trophy as a manager, with his most notable achievement thus far being a promotion with the latter from the Netherlands' second tier in 2005.
However, his transformation of Ajax after their somewhat uninspiring final season under Frank de Boer into an entertaining side that pressed high and played attractive, attacking football at a quick tempo improved his standing in the game.
Per Marca's Chris Winterburn, Manchester United boss Jose Mourinho praised his work with the side ahead of the Europa League final:
Dortmund blogger Luca Gierl is expecting him to implement a similar style in Germany:
There could be some concern over the way his Ajax side were nullified by United in the final, but at Dortmund he'll have more resources and a stronger squad to work with, so he may have the depth and quality needed to come up with a more effective Plan B when things aren't going his way.
Bosz is an exciting choice for Dortmund, but he needs now to add silverware to his cabinet—he'll have an excellent platform to do so here.