Over 18 million adolescents use the emergency department (ED) each year. High risk of unintended pregnancy is associated with frequent use of the ED and lack of a primary care provider, making the ED visit an important potential site for intervention. With few adolescent preventive health interventions being implemented and evaluated in the ED setting, the description of the rigorous development of an ED-based pregnancy prevention intervention can guide future adolescent emergency medicine researchers. The objective of this study is to describe a systematic approach, intervention mapping, to develop a theory-based, user-informed, digital intervention aiming to increase contraception use among adolescent female ED patients at high risk of unintended pregnancy. We followed the 6 iterative steps of intervention mapping to create a blueprint for designing, implementing, and evaluating an ED-based, digital, pregnancy prevention intervention: (1) Developed a Logic Model of the Problem—teen pregnancy in the ED setting—based on a needs assessment; (2) Stated the outcomes and objectives of our intervention via a Logic Model of Change, including our theoretical framework; (3) Developed the pregnancy prevention curriculum and plan, including the digital strategy and brand; (4) Produced the intervention, including designing the branched-logic texting algorithms, website, videos, and cartoons; (5) Planned program use, such as implementation strategies in the ED setting; and (6) Developed an evaluation plan. Using the process of intervention mapping, we developed Dr. Erica (Emergency Room Intervention to improve the Care of Adolescents). Dr. Erica is a relatable, empathetic, straightforward and reliable virtual female doctor who sends 10 weeks of personalized and interactive evidence-based text messages that aim to promote healthy sexual behaviors and decrease teen pregnancy risk. Texts are based on key stakeholder input, participatory design, and the Social Cognitive Theory as well as incorporate social media, relatable characters and Motivational Interviewing techniques. Following the outline of intervention mapping led to the development of a theory-based, user-informed, digital pregnancy prevention intervention. Outlining this method may be helpful to future investigators interested in designing and evaluating adolescent preventive health interventions in both the acute and outpatient setting.