The 24-year-old joins the Bundesliga side as a replacement for Pierre-Emerick Aubameyang following his big money move to Arsenal earlier in the day.
He will wear the number 44 shirt for Dortmund for the remainder of the campaign.
Despite a number of key performances, notably scoring the goal that won Chelsea the Premier League title last season, Batshuayi had struggled to prove himself to manager Antonio Conte in west London and will hope Dortmund can hep kickstart his career.
Batshuayi had scored 23 goals in all competitions for Marseille in 2015/16 prior to his €35m move to Chelsea and was considered at that time to be one of Europe's hottest new talents.
Even without regular starts, he has found the net 10 times in all competitions so far this season and also boasts a very respectable international return of five goals in 11 games for Belgium.