Negative refractive index (NRI) metamaterials with simultaneously negative permittivity and negative permeability have recently experienced tremendous fundamental and practical interest due to their unique electromagnetic properties. In this work, polythiophene (PTh) thin films were synthesized using an in situ chemical oxidative polymerization route. The double negative characteristics (negative permittivity and permeability) appeared simultaneously in the as-prepared PTh thin films. The structural and microstructural characterizations of as-deposited PTh thin films were examined through different techniques including XRD, FESEM, TEM, XPS and FT-IR spectroscopy. Furthermore, microwave absorption properties such as reflection loss (RL), shielding effectiveness (SE) of the PTh thin films with varying monomer concentration were also investigated. The morphological investigation shows that the unique porous microstructure of the PTh fibers has a dramatic impact in the enhancement kinetics of the microwave absorption performance. The PTh fibers deposited at 0.2 M concentration of thiophene showed minimum reflection loss (RL) of -44.24 dB (99.9% microwave absorption) at 17.59 GHz frequency, with a total shielding effectiveness of -18 dB at 16.3 GHz with only 5.31 microns of matching thickness. The PTh thin films exhibit exceptional microwave absorption ability at a fairly thin thickness and show a greater effective absorption bandwidth (RL < -10 dB) in the Ku-band region; these findings definitely pave the way to explore the study of different conducting polymer materials for utilization in EMI-shielding applications.