Poly(3,4-ethylenedioxythiophene)-functionalized multiwalled carbon nanotube [PEDOT-f-MWCNT] nanocomposites were prepared by in-situ chemical oxidation method using 3,4-ethylenedioxythiophene [EDOT] as monomer and f-MWCNT as nanofiller. Fourier-transform infrared-spectroscopy [FTIR] and X-ray diffraction [XRD] and field emission scanning electron microscopy [FESEM] revealed the successful formation and deposition of PEDOT on the surface of f-MWCNTs. The AC conductivity, sigma(AC), dielectric properties, namely dielectric constant, epsilon', dielectric loss, epsilon '', tangent loss and quality factor, and thermal stability of PEDOT-f-MWCNT nanocomposites were investigated in the frequency range 10 Hz to 8 MHz, and temperature ranging from 30 to 950 degrees C, respectively. The results demonstrate that sigma(AC) of PEDOT-f-MWCNT nanocomposites increases with increasing the concentration of f-MWCNT in PEDOT matrix. In particular, epsilon' and sigma(AC )values of PEDOT-15 wt% f-MWCNT nanocomposite were about 3 and 45 times larger than those of pristine PEDOT. The residual weights of PEDOT-10 wt% f-MWCNT nanocomposite and pristine PEDOT at 600 degrees C were found to be nearly 44 wt% and 1.64 wt%, respectively, showing a significant improvement in the thermal stability of the nanocomposite. The observed electrical and dielectric properties, and thermal stability of PEDOT-f-MWCNT nanocomposites depict their possibilities for various electronic applications including thin-film transistors, electrodes, and photovoltaic cells.