We present experiences of transgender women (TW) who have sex with men with SMARTtest, a smartphone app to accompany the INSTI Multiplex, a one-minute, dual blood-based HIV/syphilis rapid test. TW participants (N = 11) received 10 INSTI Multiplex tests to take home for self- and/or partner-testing and installed the SMARTtest app on their phones. The SMARTtest app aimed to support INSTI Multiplex users in correctly performing the test, interpreting the results, and connecting with care following a positive HIV or syphilis screening. After 3 months, users completed in-depth interviews on their experiences. A total of  9 TW used SMARTtest with partners. App feedback was positive, but refining is necessary. Specifically, TW reported that SMARTtest is easy to use and convenient; instructions on how to use the INSTI Multiplex presented on the app were helpful to complete procedures correctly; the most frequently used feature on SMARTtest was the information on clinics that offered confirmatory testing; and participants and their partners were not concerned about app privacy but reported that this could change if INSTI Multiplex detected an HIV-positive result. Further, participants provided recommendations on how to improve SMARTtest, and changes were mostly related to features, content, functionality, navigation, and overall "look" of the app. SMARTtest is promising to facilitate INSTI Multiplex use in TW. User feedback should be integrated in future versions.