A nanocomposite comprised of molybdenum oxide and highly conductive carbon (MoOx/Carbon) was deposited onto a screen-printed gold electrode (SPGE) to be employed as a gas sensor for the detection of formaldehyde gas. First, the carbon surface was modified by acid treatment to introduce oxygen-containing groups and promote the efficient anchorage of the molybdenum precursor by surface organometallic chemistry (SOMC). Then, once the MoOx/Carbon composite was deposited onto the SPGE, a Nafion layer was added to act as a solid-state ionic electrolyte. Fourier-transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) were used to verify the carboxylated surface of carbon after the acid treatment. Scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectrosand inductively coupled plasma optical emission spectrometry (ICP-OES) were also employed to confirm the success of the SOMC synthesis. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the interaction of the nanocomposite with formaldehyde at room temperature. The nanocomposite gas sensor showed an enhanced electrical current response when increasing the concentration of formaldehyde, with a limit of detection as low as 60 ppb and sensitivity of 5.13 μA ppm−1. Additionally, the nanocomposite sensor demonstrated high selectivity to formaldehyde when compared to other volatile organic compounds (VOCs).