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Introduction Cognitive symptoms such as ‘brain fog’ are frequent and disabling in patients with SLE. We aimed to develop and validate the Lupus Brain Fog Severity Scale (LBFSS), the first patient-reported outcome measure (PROM) for cognitive symptoms in SLE.Methods The LBFSS was developed using a multi-step international approach involving healthcare professionals and patients with SLE. Patients provided free-text descriptions of brain fog, generating candidate domains through thematic analysis (step 1). Domains were rated for relevance (step 2) and those retained converted into self-assessable items (step 3). The pilot LBFSS was iteratively refined (step 4). The finalised LBFSS was validated (step 5) in a large international SLE cohort. Psychometric evaluation included internal consistency, exploratory factor analysis and assessment of construct, convergent, known-groups and criterion-related validity.Results In step 1, 147 SLE participants across 39 countries generated 21 domains for cognitive symptoms, of which 13 were retained in consensus (step 2), converted into self-assessable items (step 3) and iteratively refined using patient and physician feedback. In step 5, validation of the LBFSS instrument in 378 patients from 36 countries demonstrated high internal consistency (Cronbach’s α=0.95) and a unidimensional structure explaining 62.2% of variance. LBFSS scores were significantly higher in participants reporting brain fog (p<0.0001), confirming known-group validity. Construct validity was supported by significant correlations with functional impact and symptom severity (ρ=0.46–0.65, p<0.0001), strong convergent validity with the Perceived Deficits Questionnaire-20 (ρ=0.84, p<0.0001) while the discriminative performance for presence versus absence of self-reported brain fog was high (area under the curve=0.85).Conclusion The LBFSS is a new, valid and feasible PROM to assess brain fog and cognitive symptoms in SLE.