Frenchman Modeste joined Dortmund to fill in for Sebastien Haller, who was diagnosed with a malignant testicular tumour. Haller had only just joined the club to replace current Manchester City goal-machine Haaland.
Modeste scored only twice in 19 Bundesliga games with Dortmund last season, one of them being a last-gasp equaliser to complete a sensational two-goal comeback and snatch a 2-2 draw against Bayern Munich in October.
The Al Ahly contract with Modeste will be for one year and may be extended for one more "according to the terms that were agreed", said the club on its website late on Monday.
Modeste may debut as early as next Friday, when Al Ahly will face Algerian USM Alger in the African Super Cup in Saudi Arabia. The Egyptian League will kick off three days later.
