A self-contained smartphone-based near infrared (NIR) colorimeter is reported for the first time by utilizing the inbuilt NIR emitter (λ ≈ 770-1000 nm) and macro-lens complementary metal-oxide semiconductor (CMOS) camera of the phone. The emitted NIR light is guided through a low-cost multimode plastic fiber to illuminate the test sample, and the phone’s CMOS camera is utilized to collect and digitize the transmitted light through the sample. An easy-to-use customized Android app has been developed to operate this ultra-low-cost (<$2) and field-portable (∼174 g) smartphone-based NIR colorimeter fabricated by polyvinyl chloride material. The performance of the instrument is evaluated by measuring the free fatty acid (FFA) percentage in the recycled palm oil samples with a 0.21% average error of detection compared to the standard titration-based assessment. FFA is a significant quality indicator of vegetable oil, generally quantified in terms of oleic acid having an absorption peak at λp ≈ 930 nm, which complies with the operating wavelength band ( λ ≈ 910-980 nm) of this instrument.