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Microfluidic systems with large surface‐to‐volume ratios and superior light transmission are used to efficiently transfer mass and convert energy, and to enhance photocatalytic reactions. Utilizing the entire solar spectrum for promoting photocatalytic reactions is highly desirable and near‐infrared (NIR) radiation, in particular, has a high transmission efficiency through common polymers and materials used to construct microfluidic devices. Herein, a reliable microfluidic system using bimodal light‐harvesting technique is reported to improve the photocatalytic efficiency of C(sp3)‐H functionalization reactions using coumarin dye (C153) and lanthanide‐doped upconversion nanocrystals (UCNs). Using two light‐harvesting components (C153 and UCNs) in polycarbosilane polymer matrix, a bimodal light‐harvesting microfluidic reactor is realized in which the inner surface of the microfluidic channel is reliably coated with a transparent composite of C153/UCNs to simultaneously downshift visible light and upconvert NIR light. A double‐stacked microfluidic system that successfully enhanced the photocatalytic conversion efficiency of Rose Bengal‐based aza‐Henry photocatalytic reactions by twofold (≈93% conversion). The study provides a design principle of next‐generation microfluidic reactor for a robust photocatalytic organic synthesis. [ABSTRACT FROM AUTHOR]