Borussia Dortmund lost from Mainz 3-0 in the 33rd round of the German League.
Dortmund suffered its seventh loss this season in the German League and remained in fifth place with 60 points.
Mainz emerged from the relegation positions by raising its score to 32 points before the final round at the end of the season.
Dortmund concludes the season by facing Darmstadt in the final round, a team that was previously relegated at Signal Iduna Park before playing the Champions League final against Real Madrid on June 1.
While Mainz still has another 90 minutes to escape the relegation struggle, as it will visit Wolfsburg in the final round.
Luxembourgian Leandro Barrero scored the lead goal in the 12th minute.
In the 19th and 23rd minutes, South Korean Lee Jae-sang scored a brace for Mainz, raising the score to 3-0.
Youssoufa Moukoko scored a goal for Dortmund in the 53rd minute, but it was ruled out for offside after returning to video technology, so the match ended in a loss for the Westphalia Lions.