The side, who finished fourth in the German top flight last season, are looking to increase their chances of closing the gap on Bayern Munich in the next and are likely to see an influx of player arrivals this summer.
Diallo, meanwhile, spent a solitary season with Mainz after a six-year stint at Monaco, where he started as a 14-year-old. He is also the captain of the France Under-21 squad.
"Borussia Dortmund has signed with Abdou Diallo, a preferred candidate for the defense," Dortmund announced via their official website
"The 22-year-old Frenchman changes from league rival Mainz 05 to BVB, at the eight-time German champions he signed a five-year contract dated to June 30, 2023."