Introduction: Patient-reported outcome measures (PROM) are questionnaires that can be used to elicit care outcome information from patients. We sought to develop and validate the first PROM for adult patients without a primary mental health or addictions presentation receiving emergency department (ED) care and who were not hospitalized. Methods: PROM development used a multi-phase process based on national and international guidance (FDA, NQF, ISPOR). Phase 1: ED outcome conceptual framework qualitative interviews with ED patients post-discharge informed four core domains (previously published). Phase 2: Item generation scoping review of the literature and existing instruments identified candidate questions relevant for each domain for inclusion in tool. Phase 3: Cognitive debriefing existing and newly written questions were tested with ED patients post-discharge for comprehension and wording preference. Phase 4: Field and validity testing revised tool pilot tested on a national online survey panel and then again at 2 weeks (test-retest). Phase 5: Final item reduction using a Delphi process involving ED clinicians, researchers, patients and system administrators. Phase 6: Validation - psychometric testing of PROM-ED 1.0. Results: Four core outcome domains were defined in Phase 1: (1) understanding; (2) symptom relief; (3) reassurance and (4) having a plan. The domains informed a review of existing relevant questionnaires and instruments and the writing of additional questions creating an initial long-form questionnaire. Eight patients participated in cognitive debriefing of the long-form questionnaire. Expert clinicians, researchers and patient partners provided input on item refinement and reduction. Four hundred forty-four patients completed a second version of the long-form questionnaire (add in retest numbers) which informed the final item reduction process by a modified Delphi method involving 21 diverse contributors. The questionnaire was validated and underwent final revisions to create the 21 questions that constitute PROM-ED 1.0. Conclusion: Using accepted PROM instrument development methodology, we developed the first outcome questionnaire for use with adult ED patients who are not hospitalized. This questionnaire can be used to systematically gather patient-reported outcome information that could support and inform improvement work in ED care.