BACKGROUND: Young men who have sex with men (YMSM) are disproportionately impacted by HIV and other sexually transmitted infections (STIs) in the United States (US) and have low rates of HIV/STI testing. Provision of HIV self-testing and STI self-collection can increase testing rates, and access to these kits through mobile applications (apps) could help facilitate YMSM using HIV self-testing and STI self-collection. METHODS: Data for this study comes from two pilot randomized controlled trials (RCTs) of mobile apps within the Adolescent Trials Network-LYNX and MyChoices-aimed to increase HIV/STI testing among YMSM (age 15-24) who had not recently tested for HIV and were at high risk for HIV acquisition across five US cities. Both apps include the ability to order a HIV self-test with rapid results and a kit for STI self-collection and mailing of samples for syphilis, gonorrhea and chlamydia to a lab for testing. Using assessments of app users (n=80) at pre-randomization and at 3- and 6-months post-randomization and online interview data from a purposive sample of app users (n=37), we report on experiences and lessons learned with HIV self-testing and STI self-collection kits ordered via the apps. RESULTS: Participants were on average 20.7 years of age (SD =2.4), and 49% were non-White or multiple race/ethnicity. Sixty-three percent had a prior HIV test. Over half (58%) had a prior STI test, but only 3% had tested within the past 3 months. Nearly two-thirds ordered an HIV self-testing kit; of whom, 75% reported using at least one self-test kit over the study period. STI self-collection kit ordering rates were also high (54%); however, STI self-collection kit return rates were lower (13%), but with a high positivity rate (5.3%). Both HIV self-testing and STI self-collection kits were highly acceptable, and 87% reported that it was extremely/very helpful to be able to order these kits through the apps. The most common reason for not ordering the HIV/STI kits was preferring to test at a clinic. In interviews, participants expressed feeling empowered by being able to test at home; however, they also raised concerns around STI sample collection. CONCLUSIONS: HIV self-testing and STI self-collection kit ordering via mobile apps is feasible, acceptable and may show promise in increasing testing rates among YMSM. The LYNX and MyChoices apps are currently being tested in a full-scale efficacy trial, and if successful, these innovative mobile apps could be scaled up to efficiently increase HIV/STI testing among youth across the US.