Xabi Alonso’s side were heavily favoured to reach their second successive final but were outclassed from start to finish by their underdog opponents.
Jonathan Tah’s 17th-minute goal was cancelled out by a Marius Woerl strike for the hosts three minutes later.
Maximilian Grosser’s goal in first-half stoppage time would prove the difference for Bielefeld, who have now beaten four top-division opponents on their way to the final.
Bielefeld, who have never made a German Cup final, became just the fourth third-division team to reach the showpiece event.
Should Bielefeld win in Berlin in May – they will face either RB Leipzig or Stuttgart – the club will be rewarded with a Europa League spot next season.
“We didn’t need luck at all because we were on the front foot the whole time,” home coach Michel Kniat said.
Kniat said he “normally wouldn’t have a drink with the players, but tonight I’ll make an exception”, adding that “nobody will go to sleep in this city tonight”.
Leverkusen’s Robert Andrich said it was “by far our worst game of the season” and added that “Bielefeld deserved the win tonight, which means we did plenty of things wrong”.
Leverkusen coach Alonso came into the game having never lost in 10 matches in the competition and needed just one more victory to beat Louis van Gaal’s all-time record set as Bayern Munich manager.
Everything seemed to go according to plan when Leverkusen took the lead after 17 minutes, Tah tapping in unmarked at the far post.
The hosts hit back almost immediately however, Woerl taking advantage of a sloppy clearance attempt by Piero Hincapie to curl the ball home.
As the half wore on, Leverkusen were unable to make their possession count, with Bielefeld fast and threatening on the counter.
The hosts once again caught Leverkusen napping to take the lead moments before halftime, Grosser hammering in a Louis Oppie free kick from close range.
Bielefeld continued to out-energise their opponents, fighting in the duels and giving Leverkusen few chances to drag themselves back into the game.
Patrick Schick, so often Leverkusen’s late hero over the past two seasons, had a chance to level the scores while unmarked after 81 minutes but hit a header against the post.
This year’s German Cup is one of the more open in recent memory, with recent champions Bayern Munich, Borussia Dortmund and Eintracht Frankfurt all suffering early eliminations from the competition.
On Wednesday, RB Leipzig play away at Stuttgart in the other semi-final.
Leipzig, who have won two of the past three German Cups, will be managed by interim coach Zsolt Low, who took over after Marco Rose was sacked on Sunday. (AFP)
