Both Real Madrid and Bellingham struggled for long periods in the final, with underdogs Dortmund squandering numerous first-half chances, including hitting the post.
But Los Blancos grew into the game after the break, culminating in late goals from Dani Carvajal and Vinicius Jr.
Bellingham spent three seasons with Dortmund, playing more than 100 times for the German club before moving to Spain last summer.
“I have to be honest, they [Dortmund] were the better team in the first half,” Bellingham told DAZN. “They could’ve been two or three up.
“But it’s just that character… [we can] come back to haunt you.
“The club I’m at now is so special to me and so is my old club. It wouldn’t be possible without them. I’ll always be grateful and always have respect for what that club did for me.”
Borussia Dortmund boss Edin Terzic congratulated his former player Bellingham after the game.
“When he left us I said the same thing that I said to Erling Haaland – that I was proud to be their manager.
“It is his first Champions League win and it is a proud moment for him.
“I know what Mark, Denise and Jobe [Bellingham’s dad, mum and younger brother] are doing to get this success in the family. Congratulations to Jude.”
Bellingham, who turns 21 at the end of June, went over to the crowd and placed his Champions League winner’s medal around his mum’s neck after the game.
She responded by kissing her son.
He then led her onto the pitch so she could have a picture with former Real Madrid boss Jose Mourinho, who was working for TNT Sports.
