A low-cost disposable colorimetric microfluidic paper-based analytical device (μPAD) was developed for the determination of nitrite and nitrate. Nitrite is determined directly by the Griess reaction while nitrate is first reduced to nitrite in a hydrophilic channel of the μPAD with immobilized zinc microparticles. This μPAD is fabricated by a simple and inexpensive inkjet printing method. Under optimal conditions, the limits of detection and quantification for nitrite are 1.0 and 7.8 μM, respectively, while the corresponding values for nitrate are 19 and 48 μM, respectively. The repeatability, expressed as relative standard deviation (RSD), is less than 2.9% and 5.6% (n ≤ 8) for the determination of nitrite and nitrate, respectively. This μPAD was successfully applied to the determination of nitrate and nitrite in both synthetic and natural water samples. It is user and environmentally friendly and suitable for on-site measurement of the analytes mentioned above in environmental and drinking waters.