Objective: Electronic (E) devices read and quantify lateral flow-based rapid tests, providing a novel approach to assay interpretation. We evaluated the performance of one E-reader for two dual HIV and syphilis immunoassays. Methods: We enrolled men who have sex with men and transgender women >18 years of age seeking medical services at an STD clinic in Lima, Peru, between October 2016 and April 2017. Venous blood was tested using two dual HIV and syphilis antibody immunoassays (SD BIOLINE HIV/Syphilis Duo, Republic of Korea, and First Response HIV 1+2/Syphilis Combo, India). Reference testing included a fourth-generation ELISA for HIV antibodies and use of the Treponema pallidum particle agglutination assay for syphilis antibodies. Trained clinic staff visually inspected the immunoassay results, after which the immunoassays were read by the HRDR-200 E-reader (Cellmic, USA), an optomechanical smartphone attachment. We calculated the concordance of the E-reader with visual inspection, as well as the sensitivity of both rapid immunoassays, in detecting HIV and T. pallidum antibodies. Results: On reference testing of 283 participant specimens, 34% had HIV antibodies and 46% had T. pallidum antibodies. Using First Response, the concordance of the E-reader with visual inspection was 97% (95% CI 94% to 99%) for T. pallidum and 97% (95% CI 95% to 99%) for HIV antibodies. Using SD BIOLINE, the concordance of the E-reader with visual inspection was 97% (95% CI 94% to 99%) for T. pallidum and 99% (95% CI 98% to 99%) for HIV antibodies. For both immunoassays, the sensitivity for HIV antibodies was 98% (95% CI 93% to 100%) and the sensitivity for T. pallidum antibodies was 81% (95% CI 73% to 87%). Conclusions: E-reader results correlated well with visual inspection. The sensitivities of both rapid assays were comparable with past reports. Further evaluation of the E-reader is warranted to investigate its utility in data collection, monitoring and documentation of immunoassay results.