A smartphone-based near ultraviolet (UV) colorimeter is demonstrated for the first time using an external UV LED source (Δλ ≈ 365 − 415 nm) and the inbuilt CMOS camera of a smartphone. The UV LED is powered directly via a micro-USB on-the-go (OTG) cable, while a customized 3D-printed PLA enclosure ensures precise alignment of optical elements and sample positioning. The transmitted UV light through the sample is captured by different smartphone cameras, and a custom app processes the image to quantify absorbance. The ultra-low-cost (∼$4) and lightweight (∼198 g) colorimeter is compatible with both Android and iOS platforms and requires no external computing hardware. Beyond local computation, the system integrates a cloud-assisted reporting feature that transmits test results, including GPS coordinates and captured images, for geospatial visualization and remote monitoring, enabling large-scale traceability of food samples. Performance of the instrument is evaluated through determination of extra virgin olive oil (EVO) adulteration in blends with refined olive oil, soybean oil, and palm oil, showing a strong correlation with commercial UV–Vis spectroscopy and an average detection accuracy of 94.98% relative to standard measurements. Cross-adulteration in cases such as palm oil in soybean oil and soybean oil in refined olive oil was also effectively identified by the device. Furthermore, pigment degradation under sunlight exposure is effectively monitored, highlighting the system’s potential in food authentication and quality monitoring.