Objective FHL1‐related reducing body myopathy is an ultra‐rare, X‐linked dominant myopathy. In this cross‐sectional study, we characterize skeletal muscle ultrasound, muscle MRI, and cardiac MRI findings in FHL1‐related reducing body myopathy patients. Methods Seventeen patients (11 male, mean age 35.4, range 12–76 years) from nine independent families with FHL1‐related reducing body myopathy underwent clinical evaluation, muscle ultrasound (n = 11/17), and lower extremity muscle MRI (n = 14/17), including Dixon MRI (n = 6/17). Muscle ultrasound echogenicity was graded using a modified Heckmatt scale. T1 and STIR axial images of the lower extremity muscles were evaluated for pattern and distribution of abnormalities. Quantitative analysis of intramuscular fat fraction was performed using the Dixon MRI images. Cardiac studies included electrocardiogram (n = 15/17), echocardiogram (n = 17/17), and cardiac MRI (n = 6/17). Cardiac muscle function, T1 maps, T2‐weighted black blood images, and late gadolinium enhancement patterns were analyzed. Results Muscle ultrasound showed a distinct pattern of increased echointensity in skeletal muscles with a nonuniform, multifocal, and “geographical” distribution, selectively involving the deeper fascicles of muscles such as biceps and tibialis anterior. Lower extremity muscle MRI showed relative sparing of gluteus maximus, rectus femoris, gracilis, and lateral gastrocnemius muscles and an asymmetric and multifocal, “geographical” pattern of T1 hyperintensity within affected muscles. Cardiac studies revealed mild and nonspecific abnormalities on electrocardiogram and echocardiogram with unremarkable cardiac MRI studies. Interpretation Skeletal muscle ultrasound and muscle MRI reflect the multifocal aggregate formation in muscle in FHL1‐related reducing body myopathy and are practical and informative tools that can aid in diagnosis and monitoring of disease progression.