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Objective: Magnetic resonance imaging can detect neurodegenerative iron accumulation in the motor cortex, called the motor band sign. This study aims to evaluate its sensitivity/specificity and correlations to symptomatology, biomarkers, and clinical outcome in amyotrophic lateral sclerosis. Methods: This prospective study consecutively enrolled 114 persons with amyotrophic lateral sclerosis and 79 mimics referred to Karolinska University Hospital, and also 31 healthy controls. All underwent 3‐Tesla brain susceptibility‐weighted imaging. Three raters independently assessed motor cortex susceptibility with total and regional motor band scores. Survival was evaluated at a median of 34.2 months after the imaging. Results: The motor band sign identified amyotrophic lateral sclerosis with a sensitivity of 59.6% and a specificity of 91.1% versus mimics and 96.8% versus controls. Higher motor band scores were more common with genetic risk factors (p = 0.032), especially with C9orf72 mutation, and were associated with higher neurofilament light levels (std. β 0.22, p = 0.019). Regional scores correlated strongly with focal symptoms (medial region vs. gross motor dysfunction, std. β −0.64, p = 0.001; intermediate region vs. fine motor dysfunction, std. β −0.51, p = 0.031; lateral region vs. bulbar symptoms std. β −0.71, p < 0.001). There were no associations with cognition, progression rate, or survival. Interpretation: In a real‐life clinical setting, the motor band sign has high specificity but relatively low sensitivity for identifying amyotrophic lateral sclerosis. Associations with genetic risk factors, neurofilament levels and somatotopic correspondence to focal motor weakness suggest that the motor band sign could be a suitable biomarker for diagnostics and clinical trials in amyotrophic lateral sclerosis. [ABSTRACT FROM AUTHOR]