Promoted VfB Stuttgart suffered a disappointing return to the Bundesliga when they lost 3-2 at home to Freiburg.
Fellow newcomers Arminia Bielefeld drew 1-1 at Eintracht Frankfurt while Werder Bremen, who narrowly avoided relegation last season, reverted to type and lost 4-1 at home to Hertha Berlin. Augsburg won 3-1 at Union Berlin and Hoffenheim beat Cologne 3-2 late on with Andrej Kramaric hitting a hat-trick.
Champions Bayern Munich laid down an ominous marker for the league by thumping Schalke 8-0 in Friday's season-opener, less than a month after lifting the Champions League to complete a second treble.
RB Leipzig host Mainz on Sunday with Wolfsburg also taking on Bayer Leverkusen.
The season, like in much of Europe, is beginning late because of coronavirus disruption but there were signs of normality returning with around 15 per cent of full capacity allowed at some games - despite infection numbers rising again in Germany.
Dortmund welcomed almost 10,000 fans and were soon on the front foot with Haaland twice going close and Jadon Sancho, also 20, clipping the woodwork.
They took the lead when English debutant Bellingham calmly played in Reyna, who finished with aplomb on 35 minutes.
Club captain Marco Reus watched on from the bench after a long injury lay-off but his younger team-mates were having no problems against his former team, despite losing Thorgan Hazard to a knock.
Gladbach had their moments but Dortmund, runners-up last term, went further in front when Haaland stroked home a 53rd-minute penalty after Reyna was tripped.
Dortmund then raced from one end of the pitch to the other with Haaland finishing off a sensational move.
Stuttgart's first game back in the top-flight after a season in the second tier was marred by poor defending, striker Nils Petersen being left completely unmarked to head in Freiburg's opener.
The visitors, who have lost first-teamers Luca Waldschmidt, Robin Koch, Alexander Schwolow and Mike Frantz to transfers, soon grabbed their second through Roland Sallai.
A well-worked move was then finished off by Vincenzo Grifo to make it 3-0 before 2007 champions Stuttgart rallied through Sasa Kalajdzic and Silas Wamangituka.
"We gave away the first two goals and in a way the third goal," Stuttgart captain Gonzalo Castro told Sky. "It happens very quickly in the top league that you can find yourself 3-0 down."
Ambitious Hertha, who lost to second-tier Eintracht Braunschweig in the German Cup first round last Friday, took the lead in Bremen when Peter Pekarik slid in their first shot on target on 42 minutes.
Dodi Lukebakio soon lashed in Hertha's second just before the break.
A swift attack led to Matheus Cunha sweeping home the third as Bremen's demons returned after a mini-revival saved them late last term. Davie Selke's towering header was a consolation with Jhon Cordoba marking his Hertha debut with a late fourth.
New Hoffenheim coach Sebastian Hoeness had a happy start to his Bundesliga career.
Kramaric scored after four minutes following an error from Jonas Hector.
Cologne levelled through their new signing from Union, Sebastian Andersson, but Kramaric struck from the spot on the stroke of half-time to boost Hoeness, nephew of Bayern icon Uli.
Cologne hit back again on 86 minutes through Dominick Drexler but Kramaric had the last laugh in injury-time.
Fellow newcomers Arminia Bielefeld drew 1-1 at Eintracht Frankfurt while Werder Bremen, who narrowly avoided relegation last season, reverted to type and lost 4-1 at home to Hertha Berlin. Augsburg won 3-1 at Union Berlin and Hoffenheim beat Cologne 3-2 late on with Andrej Kramaric hitting a hat-trick.
Champions Bayern Munich laid down an ominous marker for the league by thumping Schalke 8-0 in Friday's season-opener, less than a month after lifting the Champions League to complete a second treble.
RB Leipzig host Mainz on Sunday with Wolfsburg also taking on Bayer Leverkusen.
The season, like in much of Europe, is beginning late because of coronavirus disruption but there were signs of normality returning with around 15 per cent of full capacity allowed at some games - despite infection numbers rising again in Germany.
Dortmund welcomed almost 10,000 fans and were soon on the front foot with Haaland twice going close and Jadon Sancho, also 20, clipping the woodwork.
They took the lead when English debutant Bellingham calmly played in Reyna, who finished with aplomb on 35 minutes.
Club captain Marco Reus watched on from the bench after a long injury lay-off but his younger team-mates were having no problems against his former team, despite losing Thorgan Hazard to a knock.
Gladbach had their moments but Dortmund, runners-up last term, went further in front when Haaland stroked home a 53rd-minute penalty after Reyna was tripped.
Dortmund then raced from one end of the pitch to the other with Haaland finishing off a sensational move.
Stuttgart's first game back in the top-flight after a season in the second tier was marred by poor defending, striker Nils Petersen being left completely unmarked to head in Freiburg's opener.
The visitors, who have lost first-teamers Luca Waldschmidt, Robin Koch, Alexander Schwolow and Mike Frantz to transfers, soon grabbed their second through Roland Sallai.
A well-worked move was then finished off by Vincenzo Grifo to make it 3-0 before 2007 champions Stuttgart rallied through Sasa Kalajdzic and Silas Wamangituka.
"We gave away the first two goals and in a way the third goal," Stuttgart captain Gonzalo Castro told Sky. "It happens very quickly in the top league that you can find yourself 3-0 down."
Ambitious Hertha, who lost to second-tier Eintracht Braunschweig in the German Cup first round last Friday, took the lead in Bremen when Peter Pekarik slid in their first shot on target on 42 minutes.
Dodi Lukebakio soon lashed in Hertha's second just before the break.
A swift attack led to Matheus Cunha sweeping home the third as Bremen's demons returned after a mini-revival saved them late last term. Davie Selke's towering header was a consolation with Jhon Cordoba marking his Hertha debut with a late fourth.
New Hoffenheim coach Sebastian Hoeness had a happy start to his Bundesliga career.
Kramaric scored after four minutes following an error from Jonas Hector.
Cologne levelled through their new signing from Union, Sebastian Andersson, but Kramaric struck from the spot on the stroke of half-time to boost Hoeness, nephew of Bayern icon Uli.
Cologne hit back again on 86 minutes through Dominick Drexler but Kramaric had the last laugh in injury-time.