The application of urea-based selective catalytic reduction products (i.e., Urea-SCR) provides a reduction of NOx and, therefore, minimizes pollution emissions from vehicles fueled by diesel. Such products can be easily found in the market; however, they are often susceptible to adulteration, mainly in terms of the urea content and dilution with non-mineralized water. In this study, we propose a simple, low-cost, disposable, and straightforward paper-based microfluidic device for the quality-control of Urea-SCR products for the first time by quantifying urea and water hardness simultaneously via colorimetric reactions using a small volume of sample. 4-(dimethylamino)benzaldehyde and Eriochrome T were used as colorimetric indicators for urea and water hardness determination, respectively. Each reagent (1.5 µL) was combined with 6 µL of sample for analysis, contributing to an expressive reduction of waste generation. Digital images of the µPAD were obtained, and linear relations between color intensity and urea and Ca2+ and Mg2+ concentrations in the range of 0.2 to 1.0% and 0.1 to 3.5 mmol L−1 were obtained with a correlation coefficient higher than 0.99. Recovery experiments were employed to evaluate the accuracy of the methodology, revealing suitable values between 91.5 and 115%. Brazilian Urea-SCR samples were acquired from different distributors and submitted to the proposed procedure to evaluate its applicability. The application of microfluidic paper-based devices with colorimetric reactions enables the quality control of Urea-SCR products with high accuracy, portability, low consumption of reagents, and no generation of toxic residues; thereby contributing to the green analytical chemistry field.