Background Given the steady increase of emergency department (ED) visits related to opioid overdoses, this study aims to determine the design and usability of an ED-centered mHealth patient-to-peer referral prototype tool that allows patients to refer peers to comprehensive HIV/HCV and opioid misuse prevention services. Methods Two iterative focus group discussion (FDG) sessions and one use-case session were conducted. Eligible participants who were ≥18 years, had a history of injection drug use (IDU), and had utilized the ED in the past year were recruited through the distribution of flyers at the study institution, including the study ED. Human-centered design process was completed by using participant feedback on perceived utility, usability/accessibility, tool design, and clarity/readability to fine-tune prototype version and drive subsequent discussion sessions. Results Sixteen consented individuals participated in at least one of the sessions. Feedback revealed that participants favored the inclusion of the webpage link on the referral card as means to bypass QR code if needed, more descriptions highlighting the exact services offered, and the fact that no personal information was required to complete the referral process. The prototype underwent several adjustments between user-centered FDG sessions, which ultimately ended in including features such as an online webpage with educational videos, SMS text-message communication system, and QR code usage into the final patient-to-peer referral tool prototype. Conclusion The findings of this study suggest a human-centered designed patient-to-peer referral tool could be a feasible approach to linking community members at risk of IDU to HIV/HCV and opioid use-related preventive services from ED patients.