Phenolic compounds derive from the secondary metabolism of plants and present several biological functions as well as antioxidant activity. Among the spectrophotometric methods, the reaction involving reduction of the Folin-Ciocalteu reagent by phenolates is the most used for the analytical determination. Conventional methods are time-consuming and require high amounts of reagents and sample, thus generating large waste amounts. These hindrances are particularly critical due to the high demand for this analytical task. The aim of this work was to develop an analytical alternative to determine phenolic compounds from acerola cherry (Malpighia emarginata) exploiting a greener microwave-assisted extraction and photometric detection using a smartphone camera as detector. Microwave-assisted extractions were optimized by Doehlert design, in which temperature (from 30 to 60 °C), solvent composition (from 0 to 99% v/v ethanol), and extraction time (from 10 to 50 min) were evaluated. Total phenolic content was determined by the Folin-Ciocalteu method performed on Eppendorf^® tubes and taking the intensity of reflected radiation as analytical signal. The response at R channel was considered to this aim because of the complementarity with the color of the product. The images were acquired under controlled illumination, inside a styrofoam box containing a LED lamp in the bottom, using the camera of a smartphone Xiaomi Mi A3 positioned perpendicularly to the sample, with a region of interest of 32 x 32 pixel. Images were processed in ColorGrab app (Loomatix®, version 3.6.1, 2017). The best condition for the extraction was 66% v/v ethanol, at 38 °C, for 50 min. Total phenolic compounds was determined by the proposed procedure, corresponding to 146 ± 4 mg GAE g^-1 dry weight, which agreed with the reference procedure (145.0 ± 0.7 mg GAE g^-1 dry weight) at the 95% confidence level. A five-fold reduction in the reagent amount was achieved.