Veteran midfielder Sebastian Hille was the matchwinner, heading in the decisive goal in the 56th minute to give his side all three points.
The win moved Bielefeld level on 76 points with league leaders Kalrsruher SC, who likewise secured promotion on Matchday 37. Both sides boast exactly the same balance sheet, having won 22 matches this season, drawing ten and losing five.
However, Karlsruhe are top due to their superior goal difference. Third placed 1. FC Heidenheim are five points further behind in the play-off spot, with just one more game left to play.