PURPOSE: The Adelphi Adherence Questionnaire (ADAQ©) is a newly developed generic patient-reported outcome (PRO) assessment of medication adherence. The aim was to assess its content validity by conducting cognitive debriefing (CD) interviews with patients prescribed medication(s) of various treatment modalities in a range of therapy areas. MATERIALS AND METHODS: Targeted literature/instrument review and concept elicitation interviews informed development of the ADAQ©. CD interviews were conducted with 57 adults from the United States of America (USA; n = 21), Spain (n = 18), and Germany (n = 18) who prescribed medication for hypertension, diabetes, depression, schizophrenia, asthma, multiple myeloma, psoriasis, and/or multiple sclerosis. Interviews were conducted in two rounds to explore the relevance and understanding of the item wording, instructions, recall period and response options. Verbatim transcripts were analysed in ATLAS.Ti using thematic analysis. Three expert clinicians provided guidance throughout the study. RESULTS: ADAQ© items/instructions were well understood and relevant to participants. Key modifications following round 1 included revising instructions to refer to current medication(s) for one condition to reduce cognitive burden, removing two items with lower relevance (specifically those assessing running out of medication and social discouragement), and adding a response option for participants to indicate if they had stopped taking a medication. Minor wording modifications were made following round 2. Subgroup differences in item relevance were explored based on clinical characteristics. Cost of medication was more relevant amongst US participants. CONCLUSION: Content validity of the ADAQ© was confirmed in demographically and clinically diverse participants. Psychometric properties of the ADAQ© will be explored in future studies.