In this research, we present the first successful preparation of polyaniline/gamma-alumina nanocomposites towards their use in anticorrosive coatings. Polyaniline/gamma-alumina nanocomposites have been successfully synthesized via in situ ultrasonic dispersion and oxidative polymerization, and then characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The coatings of polyaniline/ gamma-alumina nanocomposites on a cold-reduced carbon steel sheet were found to provide greatly superior corrosion protection compared with pure polyaniline, through a series of electrochemical measurements in 5 wt% aqueous NaCl electrolyte. As the loading of gamma alumina increased, the corrosion potential became less negative, the polarization resistance increased, and the corrosion current decreased, indicating that the anticorrosion properties were improved. By electrochemical impedance spectroscopy (EIS) analyses, the results of Nyquist plots were fitted using an equivalent circuit, which presents two characteristic semicircles with marked increases in the pore resistance (Rpore) and charge transfer resistance (Rt) of nanocomposites along with the loading of gamma alumina. Bode plots also showed a great increase in impedance for the coatings with the loading of gamma alumina. The effect of the corrosion inhibition of the nanocomposite coating was also evidenced by scanning electron microscopy (SEM) after the corrosion test.