We report the first case of blood chimerism involving a pathogenic RB1 variant in naturally conceived monochorionic‐dizygotic twins (MC/DZ) with the twin–twin‐transfusion syndrome (TTTS), presumably caused by the exchange of stem‐cells. Twin A developed bilateral retinoblastoma at 7 months of age. Initial genetic testing identified a de novo RB1 pathogenic variant, with a 20% allelic ratio in both twins' blood. Subsequent genotyping of blood and skin confirmed dizygosity, with the affected twin harboring the RB1 pathogenic variant in skin and blood, and the unaffected twin carrying the variant only in blood.