The Bundesliga club will receive a transfer fee of three million euros for the 27-year-old Malian international, who departs German football after a lengthy spell within the Red Bull football structure.
Haidara's switch to France marks the first time in his professional career that he has left the "RB system". Prior to his move to Leipzig, the midfielder represented FC Liefering and RB Salzburg, before spending eight seasons with Leipzig in the Bundesliga.
During that period, he made 222 competitive appearances for the club, establishing himself as a reliable squad player across domestic and European competitions.
Despite his long-standing association with Leipzig, Haidara did not play a single minute for the club during the current season.
Nevertheless, his international standing remained intact, with the midfielder named in Mali's squad for the ongoing Africa Cup of Nations.
Haidara featured as a substitute in Mali's opening group-stage match of the tournament, which ended in a 1-1 draw against Zambia. His inclusion underlined his continued importance at international level, even amid limited club action.
"Doudou was a very important player for RB Leipzig for many years," RB Leipzig sporting director Marcel Schafer noted in a club statement.
"The last six months which were not easy for him personally and he once again proved his impeccable character by never giving up and behaving in an exemplary manner.
"We therefore wish him every success, both professionally and personally, in the Africa Cup of Nations and his next step at RC Lens."