Colorimetric determination of proteins in serum is proposed based on the biuret method and replacing ultraviolet-visible spectrometric (UV-Vis) detection with a simple and affordable smartphone digital image colorimetric (SDIC) method. Optimum SDIC conditions were found as a detection wavelength of 555 nm, a region of interest of 1600 px2, and 9.0 cm between the detection camera and sample solution. Under the optimum conditions, the coefficient of determination was 0.9982 within a linear dynamic range of 0.022 to 0.35 g dL-1. The precision of the method based on the percent relative standard deviation was below 5%. The limit of detection and limit of quantitation were found to be 0.007 and 0.022 g dL-1 respectively, which were sufficient for the quantification of the total protein, albumin, and globulin in serum. The method was validated with an independent experiment using a UV-Vis method and both methods showed good statistical agreement, indicating the accuracy of the proposed SDIC method.