The detection of ultratrace analytes is highly desirable for the non‐invasive monitoring of human diseases. However, a major challenge is fast, naked‐eye, high‐resolution ultratrace detection. Herein, a rectangular 3D composite photonic crystal (PC)‐based optoelectronic device is first designed that combines the sensitivity‐enhancing effects of PCs and optoelectronic devices with fast and real‐time digital monitoring. A crack‐free, centimeter‐scale, mechanically robust ellipsoidal composite PCs with sufficient hardness and modulus, even exceeding most plastics and aluminum alloys, are developed. The high mechanical strength of ellipsoidal composite PCs allows them to be hand‐machined into rectangular geometries that can be conformally covered with the centimeter‐scale flat light‐detection area without interference from ambient light, easily integrating 3D composite PC‐based optoelectronic devices. The PC‐based device's signal‐to‐noise ratio increases dramatically from original 30–40 to ≈60–70 dB. Droplets of ultratrace analytes on the device are identified by fast digital readout within seconds, with detection limits down to 5 µL, enabling rapid identification of ultratrace glucose in artificial sweat and diabetes risk. The developed 3D PC‐based sensor offers the advantages of small size, low cost, and high reliability, paving the way for wider implementation in other portable optoelectronic devices. A photonic crystal‐based optoelectronic device is developed for rapid and real‐time digital monitoring of ultratrace analytes with detection limits down to 5 µL, which is highly desirable for the non‐invasive monitoring of human diseases, such as the quick identification of diabetes from ultratrace glucose in sweat.