Malnutrition and sarcopenia are highly prevalent and clinically impactful conditions in patients undergoing maintenance hemodialysis (MHD), yet their early detection remains challenging. This study aimed to assess the diagnostic performance of nutritional ultrasonography (NUS) in the morphofunctional evaluation of malnutrition and sarcopenia, and to compare its utility with established tools such as bioimpedance analysis (BIA), biochemical markers, handgrip strength (HGS), and functional performance tests. A cross-sectional study was conducted in 74 stable MHD patients. Clinical, analytical, anthropometric, BIA, NUS, and functional parameters were collected, along with validated nutritional and frailty scales. NUS was used to assess the quadriceps rectus femoris (QRF) and preperitoneal visceral fat (PPVF), measuring -axis, -axis/height, cross-sectional muscle area rectus femoris (CS-MARF), muscle area rectus femoris index adjusted to height (MARFI), and supramuscular fat (SMF). Sarcopenia was defined according to the 2019 European Working Group on Sarcopenia in Older People (EWGSOP) criteria. The prevalence of risk, confirmed, and severe sarcopenia was 24.3%, 40.5%, and 20.3%, respectively. Severe-to-moderate protein-energy wasting (PEW) affected 44.6% of patients. Compared to non-sarcopenic individuals, sarcopenic patients had lower values of HGS, prealbumin, lean body mass, and phase angle. NUS-derived cut-off values for sarcopenia were -axis ≤ 8 mm, -axis/height ≤ 2.9 mm/m, CS-MARF ≤ 2.4 cm, and MARFIh ≤ 0.9 cm/m. The most discriminative NUS parameters were -axis and SMF (AUC 0.67), followed by -axis/height (AUC 0.65) and MARFI (AUC 0.63). NUS measurements correlated significantly with ASMI, phase angle, HGS, and SPPB scores. Conclusions: Nutritional ultrasonography is a feasible, reproducible, and clinically valuable tool for assessing muscle mass and quality in MHD patients. Its incorporation into routine practice may enhance early detection of malnutrition and sarcopenia, thereby facilitating timely, individualized nutritional interventions.