Abstract Background: Quitxt is an evidence-based mobile text messaging service for smoking cessation promoted in South Texas via social media advertising. A group analysis of 798 Quitxt participants with a mean age of 29.3 found that 21% of the enrollees reported abstinence at 7 months. This confirmed that a text and mobile media service specifically designed for young adults provides a feasible, potentially cost- effective approach to promoting cessation. Quitxt is tailored to reach Latino, non- Latino white and African American young smokers. Objective: We present the design, development process and preliminary results of a theory-based, bilingual, interactive Facebook Messenger Chat to promote smoking cessation among young adults in Texas. Methods: We conducted focus groups (2) and an online survey (200) with young adult smokers aged 18-30 to assess images, video testimonials, and messages for social media and updated webpages linked to in Quitxt text messages. Feedback from focus group and online survey participants informed the development of the Facebook Messenger Chat prototype. The Chat was pretested multiple times to identify and fix technical issues and improve its functionality. Results: Formative research participants positively reviewed the images, messages, and videos within the Quitxt mobile program, with the suggested addition of more information on negative consequences of smoking. Participants listed Facebook as the most popular social media platform, adding credibility to the conversion of the Quitxt text messaging program into a Facebook Messenger Chat version. This platform also enabled additional graphic content to make the messages more appealing to young adults. After beta-testing, we refined the Messenger Chat prototype and resolved technical issues. The Quitxt communication team designed Facebook ads with a “quit smoking for a New Year’s resolution” theme and began enrollment in late December 2019. Based on past social media recruitment, we expected to recruit up to 300 participants during the 2-week campaign, but we exceeded expectations and enrolled 2,347 in the Messenger Chat version of Quitxt. Additional promotional campaigns in 2020 for Valentine’s Day, St. Patrick’s Day, and as a preventive health measure amid the rise of COVID-19 produced a total of 5,344 enrollments. 73.8% were Spanish speakers, 55.2% were aged 18-29; 74% were males, 81.7% self- identified as Hispanic/Latino, most were light/medium smokers (mean 11.8 cig/day), and 51% were ready to quit the next day. About 14% reported cessation at 1 month. Conclusions: We followed an iterative design process to develop a bilingual, culturally tailored and interactive Facebook Messenger Chat prototype to deliver our Quitxt smoking cessation program. Preliminary results showed that the Facebook Messenger Chat version may be a successful strategy to deliver smoking cessation services to young adult smokers. The anticipated outcome is a scalable, evidence- based, easily disseminated smoking cessation intervention. Citation Format: Alfred L. McAlister, Edgar Muñoz, Cliff Despres, Pramod Sukumaran, Illeana Tiemann, Bianca Gutierrez, Sahak Kaghyan, Rahul Mundlamuri, David Akopian, Amelie G. Ramirez, Patricia Chalela. mHealth for smoking cessation: Development of a bilingual messenger chat to engage young adult smokers [abstract]. In: Proceedings of the AACR Virtual Conference: Thirteenth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2020 Oct 2-4. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(12 Suppl):Abstract nr PO-008.