Background and Objectives: The diagnosis and continuous care of chronic conditions such as HIV present potential teachable moments for delivering smoking prevention and cessation interventions for patients. During our study, we designed and pre-tested a prototype of a smartphone application (app), Decision-T, specifically designed to assist physicians and non-physicians when providing personalized smoking prevention and cessation services to their patients.    Methods: We developed the Decision-T app based on the transtheoretical algorithm for smoking prevention and cessation following the 5 As model. Our study used a mixed-methodsapproach among 18 HIV care providers recruited from the Houston Metropolitan Area for pre-testing the app. Each participant took part in three mock sessions using the Decision-T app. The average time spent by the provider at each mock session was measured. We also measured accuracy by comparing the smoking prevention and cessation treatment offered by the HIV care provider using the app to the treatment approach chosen by a tobacco specialist who designed the case. The system usability scale (SUS) was used to assess usability quantitatively. Transcripts of individual interviews with study participants were analyzed to assess usability qualitatively. STATA 17/SE and NVivo Version 12 were used for quantitative and qualitative analysis, respectively.    Results: The average time for completing each mock session was 5 minutes 17 seconds. The participants achieved an overall average accuracy of 88.89%. The average SUS score achieved was 87.5 (±10.26). A total of five major themes (i.e., the apps contents are beneficial and straightforward, apps design is easy to understand, apps user experience is uncomplicated, apps tech is intuitive, app needs improvements) emerged after analyzing the qualitative interviews.     Conclusions: The Decision-T app has the potential to increase HIV care providers engagement in offering smoking prevention and cessation behavioral support and pharmacotherapy recommendations to their patients briefly and accurately.