Braunschweig, Bundesliga champions in 1967 and relegated in 1985, join Hertha Berlin in winning automatic promotion.
"It is an amazing day," coach Torsten Lieberknecht told reporters as celebrations kicked off with a beer shower on the pitch. "The tears we shed will not be the last ones."
Braunschweig have 65 points, one fewer than Hertha. Third-placed Kaiserslautern, on 52, have four games to play.
Damir Vrancic scored the only goal with a last-gasp free kick.