On 12 May 2016, Dembélé signed a five-year contract with German club Borussia Dortmund, effective 1 July. On 14 August 2016, he made his debut in a 2–0 defeat against Bayern Munich in the DFL-Supercup. He scored his first goal for "die Borussen" on 20 September 2016, in a Bundesliga encounter against VfL Wolfsburg, which Dortmund won 5–1 at the Volkswagen Arena. On 22 November 2016, he scored the first Champions League goal of his career as the German club defeated Legia Warsaw 8–4 in a group stage meeting.