Detection of biomarkers in biosystems plays a key role in advanced biodiagnostics for research and clinical use. Design of new analytical platforms is challenging and in demand, addressing molecular capture and subsequent quantitation. Herein, we developed a label-free electrochemical sensor for CD44 by ligand-protein interaction. We assembled carbon nanotube composites on the electrode to enhance electronic conductivity by 6.2-fold and reduce overpotential with a shift of 77 mV. We conjugated hyaluronic acid (HA) to the surface of carbon nanotubes via electrostatic interaction between HA and poly(diallyldimethylammonium chloride) (PDDA). Consequently, we performed direct electrochemical sensing of CD44 with a dynamic range of 0.01-100 ng/mL and detection limit of 5.94 pg/mL without any postlabeling for amplification, comparable to the best current results. The sensor also displayed high selectivity, reproducibility with relative standard deviation (RSD, n = 5) of 2.57%, and long-term stability for 14 days. We demonstrated applications of the sensor in detection of human serum and cancer cells. Our work guides the development of more sensor types by ligand-protein interactions and contributes to design of interfaces in given biosystems for diagnosis.