BACKGROUND: Filamin-C (FLNC) gene variants are associated with cardiac and skeletal muscle diseases including a clear role of loss-of-function variants in dilated cardiomyopathy. OBJECTIVE: To assess the contribution of rare FLNC variants to hypertrophic/restrictive cardiomyopathy (HCM/RCM). METHODS: Family-based studies in two specialist services, and statistical modelling of rare FLNC missense variants, using a cohort of 3,289 sarcomere-negative HCM cases and 122,348 genome aggregation database controls. RESULTS: Clinical evaluation of patients with HCM/RCM and a rare FLNC variant identified a distinct ECG repolarisation phenotype in 37% (19/51 individuals, from 12 families) which was observed in only 1.0% (2/197) of a control HCM cohort. FLNC variant carriers with the characteristic ECG had smaller LV cavity size, lower contractility, more severe diastolic dysfunction, and were more likely to have a restrictive phenotype. Heart failure death, transplant or cardiac arrest occurred in at least one individual in seven of the 12 families (58%) in the 'ECG positive' group, and musculoskeletal abnormalities were present in four families (33%). Five of 12 variants (41.7%) in the 'ECG positive' group co-segregated, and two were apparently de novo. Eleven variants were missense, one splice site. Rare FLNC missense variant burden indicated a low case excess amongst all HCM cases (etiological fraction 0.45, 95% CI [0.36-0.54]), but in 'ECG positive' cases the etiological fraction was substantially higher (0.98, 95% CI [0.97-0.99]). CONCLUSION: Pathogenic FLNC variants in patients with HCM/RCM are non-truncating and cause a discrete phenotype comprising a characteristic ECG, hypertrophic and restrictive features without hypercontractility, and extra-cardiac abnormalities.