The English midfielder, who this month followed older brother Jude’s footsteps in joining the German side, started for the first time for Dortmund.
Sundowns of South Africa took a surprise early lead, but Dortmund hit back strongly to triumph in sweltering conditions at the TQL Stadium in Cincinnati and move to the top of Group F.
“It’s a nice bonus (to score), I’m glad we won, but there are a lot of things for me to improve on personally and for the team,” Bellingham told DAZN.
Dortmund coach Niko Kovac had claimed teams from the “south” had an advantage because of the heat, and the Sundowns’ 11th-minute opener appeared to confirm his fears.
Lucas Ribeiro’s fine solo goal put the CAF Champions League runners-up ahead, with the Brazilian charging forward from his half before beating Gregor Kobel.
However, Sundowns goalkeeper Ronwen Williams handed Dortmund their equaliser on a plate, passing the ball straight to Felix Nmecha, who stroked home.
Both stoppers made good saves in an end-to-end clash before Dortmund claimed the lead through striker Serhou Guirassy.
Dortmund won the ball high up the pitch and Julian Brandt crossed for the Guinea international to score with a fine leap and header, continuing his fine form this season.
Bellingham, who joined earlier in June from Sunderland for 33 million euros ($37 million), netted Dortmund’s third just before the break.
Williams parried a cross into his path, and after controlling it on his chest, Bellingham drilled home.
“I’ve practised that so many times, arriving late on the edge of the box as a midfielder is something you have to be good at,” said Bellingham.
“As a kid and at Sunderland, I’ve practised that so many times, during training, after training, so I’m proud of it.”
