Background/aim: Sarcopenia and dementia are growing concerns among older adults that muscle and brain atrophy may cooccur. We aimed to compare the age-related loss of muscle mass by using ultrasound (US), and skeletal muscle mass index (SMI) by bioelectrical impedance analysis in older adults with and without dementia. Materials and methods: A total of 221 older adults aged ≥65 years were included in the study. The diagnosis of sarcopenia was established if low muscle mass according to either SMI or sonographic gastrocnemius (GC) muscle thickness was combined with low grip strength. The diagnosis of dementia was based on the National Institute of Aging and Alzheimer’s Association criteria and the major neurocognitive disorder definition in the Diagnostic and Statistical Manual of Mental Disorders-V. Muscle strength was measured by hand dynamometer and physical performance was assessed by 4-meter usual gait speed. Results: There were similar/moderate correlation coefficients between GC muscle thickness and SMI with functional parameters (all p < 0.01). Forty-six patients (20.8%) had dementia, and 21 (45.7%) of them had sarcopenia diagnosed by GC thickness (p < 0.001). Age was older but weight, body mass index, and all sarcopenia-related parameters were lower in dementia patients (all p < 0.01). When clinical variables were taken into binary logistic regression analyses, age [OR = 1.095 (95% CI: 1.028–1.167)], weight [OR = 0.918 (95% CI: 0.887–0.950)], and presence of dementia [OR = 5.109 (95% CI: 2.002–13.033)] were independently associated with sarcopenia diagnosed with GC muscle thickness (all p < 0.05). Conclusion: This study showed that sarcopenia is highly prevalent in older adults with dementia (45.7%) than without dementia (11.4%). Amongst different factors, increased age, having low body weight, and the presence of dementia independently increased the risk of sarcopenia diagnosed by GC muscle thickness (but not diagnosed by SMI) in older adults. Thus, we can evaluate easily and successfully the loss of (regional) muscle mass in dementia patients by using US in outpatient clinics.