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[Background and Aims] Diagnosis of metabolic dysfunction-associated steatohepatitis (MASH) requires histology. In this study, a magnetic resonance imaging (MRI) score was developed and validated to identify MASH in patients with metabolic dysfunction-associated steatotic liver disease (MASLD). Secondarily, a screening strategy for MASH diagnosis was investigated., [Methods] This prospective multicentre study included 317 patients with biopsy-proven MASLD and contemporaneous MRI. The discovery cohort (Spain, Portugal) included 194 patients. NAFLD activity score (NAS) and fibrosis were assessed with the NASH-CRN histologic system. MASH was defined by the presence of steatosis, lobular inflammation, and ballooning, with NAS ≥4 with or without fibrosis. An MRI-based composite biomarker of Proton Density Fat Fraction and waist circumference (MR–MASH score) was developed. Findings were afterwards validated in an independent cohort (United States, Spain) with different MRI protocols., [Results] In the derivation cohort, 51% (n = 99) had MASH. The MR–MASH score identified MASH with an AUC = .88 (95% CI .83–.93) and strongly correlated with NAS (r = .69). The MRI score lower cut-off corresponded to 88% sensitivity with 86% NPV, while the upper cut-off corresponded to 92% specificity with 87% PPV. MR–MASH was validated with an AUC = .86 (95% CI .77–.92), 91% sensitivity (lower cut-off) and 87% specificity (upper cut-off). A two-step screening strategy with sequential MR–MASH examination performed in patients with indeterminate-high FIB-4 or transient elastography showed an 83–84% PPV to identify MASH. The AUC of MR–MASH was significantly higher than that of the FAST score (p < .001)., [Conclusions] The MR–MASH score has clinical utility in the identification and management of patients with MASH at risk of progression.