He's their third signing of the day alone, following the €25m arrival of Thorgan Hazard (Eden's brother) from Borussia Monchengladbach, and Hoffenheim full-back Nico Schulz.
Brandt, 23, lured from Bayer Leverkusen, joins new signing Thorgan Hazard, the younger brother of Chelsea star Eden, at the club who also snapped up German global defender Nico Schulz, in a transfer blitz.
Speaking on his switch, Eden Hazard's younger brother believes this was the ideal time for him to make the move.
The Belgium global has signed a five-year deal with the Bundesliga runners-up, after five years with Gladbach.
"I am proud to play for Borussia Dortmund".
"Now was exactly the right point in time to take the next step in my career".
I'm a person for whom gut instinct and having a good feeling is important when it comes to decisions.
Brandt and Hazard are joined at Dortmund by Schulz, a full-back who is beginning to establish himself as a first choice global, and arrives from Hoffenheim in a deal worth 27 million euros.
Brandt's signing has now rounded off a very busy week for Dortmund, and there could yet be more activity before the summer transfer window slams shut. I want to continue my personal development.
"He is still a young player with great development potential", Dortmund's sporting director, Michael Zorc, said of Brandt. BVB just narrowly missed out on the title last season.