Objective Medication nonadherence is common in systemic lupus erythematosus (SLE) and negatively impacts outcomes. To better recognize and address nonadherence, there is need for an easily implemented tool with interpretable scores in this population. Domains of Subjective Extent of Nonadherence (DOSE-Nonadherence) is a measure that captures both extent of and reasons for nonadherence. We refined and evaluated DOSE-Nonadherence for patients with SLE. Methods We refined the reasons for nonadherence domain of DOSE-Nonadherence through rheumatologist feedback and patient cognitive interviewing. We then administered the refined instrument to patients prescribed oral SLE medications and compared its results to the Beliefs about Medicines Questionnaire (BMQ), the Medication Adherence Self-Report Inventory (MASRI), medication possession ratios (MPRs), and hydroxychloroquine blood levels via Pearson correlations. Results Five rheumatologists provided feedback; 16 patients (median age 43, 100% female, 50% Black) participated in cognitive interviews; 128 (median age 49, 95% female, 49% Black, 88% on anti-malarials, and 59% on immunosuppressants) completed the refined instrument. Items assessing extent of nonadherence produced reliable scores (alpha 0.89) and identified 47% as nonadherent. They showed convergent validity with MASRI (r=-0.57), hydroxychloroquine blood levels (r=-0.55), and to a lesser extent MPRs (r=-0.34 to -0.4), and discriminant validity with BMQ domains (r=-0.27 to 0.32). Nonadherent patients reported on average 3.5 adherence barriers, the most common being busyness/forgetting (62%), physical fatigue (38%), and pill fatigue (33%). Conclusion Our results support the reliability and validity of DOSE-Nonadherence for SLE medications. This instrument can be used to identify, rigorously study, and guide adherence intervention development in SLE.