The 18-year-old scored Sunderland's opener in his first match against his boyhood club since he swapped the Blues for the Black Cats in June.
Bellingham followed in his Real Madrid star sibling's footsteps by coming through the St Andrew's academy.
And while Birmingham may not have retired his shirt as well, the teenager ensured he'd made his mark against his old club.
Sunderland are looking to return to winning ways ahead of the international break after being held to a goalless draw at Swansea last weekend.
The home side took just 17 minutes to go ahead against Birmingham, with a corner from former Spurs man Jack Clarke causing havoc.
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Nazariy Rusyn managed to flick it on and Bellingham reacted quickest to slam home from close-range.
The instinctive poacher's finish was reminiscent of the type of goal his older brother Jude has been making his trademark in Madrid.
Bellingham wheeled away in celebration but his antics were slightly reserved against his former club.
The midfielder made 26 first-team appearances in all competitions for boyhood club Birmingham before sealing a £3million move to Sunderland this summer.
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He has emulated his older sibling in making a fast start to life at his new club, already becoming a key player for Tony Mowbray's side.
Bellingham has now scored three goals and provided one assist in 16 appearances in all competitions.
He was name-checked by Birmingham boss Wayne Rooney ahead of today's game, who claimed the starlet would get no special treatment.
Rooney said: "I know of him but I don't know him as a person.
"I'm sure people will have fond memories of him around the club, but ultimately he's a Sunderland player and we want to beat them on Saturday. It happens all the time in football. Players move on.
"There will be no special treatment, we will prepare for him in the same way as we prepare for the rest of their players. I can't speak on why he left the club, whether that was a club decision or his decision, I don't know.
"My focus is on the players who are here. He's a bright talent and I'm sure he has a bright future, but my focus is purely on the players here."
Bellingham's goal had increased the pressure on Rooney, who is still waiting to secure his first victory in charge of his new club.
But Koji Miyoshi's well-taken finish from Jay Stansfield's cross from the right hauled Birmingham level at the break.