Lucien Favre's side arrived at Allianz Arena with a two-point lead at the top of the table but the momentum behind their title bid was ripped away in a devastating first half in the 100th league edition of Der Klassiker.
Mats Hummels made the first incision with a commanding header and Lewandowski, a fellow former Dortmund star, reached his scoring milestone with a superb solo effort seven minutes later.
Javi Martinez and Serge Gnabry assured Bayern's ascendancy to top spot by adding two more in almost as many minutes late in the opening period and Poland international Lewandowski fittingly completed the rout in the 89th minute as the champions took a big step towards retaining their crown.
A goalscoring dynasty pic.twitter.com/Udp1M7IAN1— Bundesliga English (@Bundesliga_EN) April 6, 2019
Mahmoud Dahoud wasted an early chance from Marco Reus' cut-back and he was made to rue the miss when Hummels emphatically met Thiago Alcantara's corner in the 10th minute.
Lewandowski soon became the first non-German player to notch 200 Bundesliga goals by seizing on Dan Axel-Zagadou's turnover, dinking the ball over the onrushing Roman Burki and volleying into an empty net.
Dortmund were being sliced open with regularity and, despite a series of Burki saves, were out of the game by half-time.
Martinez curled through a clutch of bodies in the 41st minute and Gnabry continued the carnage by nodding in Thomas Muller's cross.
Dortmund stemmed the bleeding after the break but never threatened the kind of comeback that saw them win 3-2 when the teams met in November.
Instead, it was left to Lewandowski to tap in Gnabry's pass late on and put the seal on a result that will ensure Bayern are brimming with confidence for the final six games.
What does it mean? Bayern land big psychological blow
Favre insisted before the match that the result of this encounter would not determine the fate of his team's title challenge but it is hard to see how a side that wobbled under pressure in February can bounce back from such a morale-sapping loss.
Hummels head and shoulders above
Bayern's nervy 5-4 victory over second-tier Heidenheim in the DFB-Pokal in midweek prompted head coach Kovac to urge his players to show "responsibility and discipline".
Hummels answered the call. The centre-back did his defensive duties with consummate ease and was a game-changing influence in the opposition box, with his opening goal just one of a number of instances in which he made himself a nuisance.
Zagadou error sparks avalanche
Dortmund were under relentless pressure amid the one-sided first 45 minutes and they were done no favours by defender Zagadou's unforgiveable error for the second goal.
The 19-year-old gave the ball directly to an in-form Lewandowski and it was no surprise when Favre replaced him with Julian Weigl at the interval.
Key Opta Facts:
- Robert Lewandowski reached the mark of 200 goals in Germany's top-flight (now 201), behind only Gerd Muller (365 goals), Klaus Fischer (268), Jupp Heynckes (220) and Manni Burgsmuller (213).
- With his first goal against Dortmund, Lewandowski also became the first player in Bundesliga history to score in five consecutive matches against a former club.
- Mats Hummels, who scored the first goal of the match, fired four shots – as many as all BVB players combined.
- Bayern have scored 20 goals in the last four home matches against Dortmund in the Bundesliga (five per game), while conceding only two.
- Dortmund conceded their biggest defeat since losing 6-0 in Munich on matchday 28 in 2017-18 and also the biggest loss since Lucien Favre became coach.