The club sold United States star Christian Pulisic to Chelsea for $73 million during the typically subdued January window, and has now bought Hazard, Brandt and left back Nico Schulz from TSG Hoffenheim for a reported total of $86 million.
Initially a winger, he thrived in the latter part of the past season after being moved into a more central midfield role by Bosz, his seven goals and 11 assists helping Leverkusen clinch a Champions League spot.
"I am grateful for five great years at Borussia Mönchengladbach and very nice memories", he told BVB's official website.
I'm a person for whom gut instinct and having a good feeling is important when it comes to decisions. "I am proud to play for Borussia Dortmund - a top club with incredible fans", Hazard said.
"Now was the flawless time to take the next step in my career".
Hazard, a Belgium midfielder who had a year to run on his contract at Borussia Moenchengladbach, scored 13 goals in 35 games last season.
"And I have a very good feeling with Borussia Dortmund".
'I am very motivated and optimistic about my personal development, but also about the whole team'.
He said: "Although Julian Brandt has played in the Bundesliga for a long time and has a lot of experience both nationally and internationally, he is still a very young professional with potential for development at the age of 23".
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.