Fast-growing flexible and stretchable electronics, such as robots, portable electronics and wearable devices, are regarded as the next-generation electronic devices. Flexible or even stretchable electromagnetic interference (EMI) shielding materials with high performance are needed to avoid the adverse effects of electromagnetic radiation produced by these devices. In this work, highly conductive and stretchable polymer films were prepared by blending a conductive polymer, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), with highly stretchable waterborne polyurethane (WPU). The two polymers have good miscibility at a wide range of blending ratios. The conductivity of the composite films increases while the stretchability decreases with the increase of PEDOT:PSS loading. At a 20 wt% PEDOT:PSS loading, the composite films show a conductivity of 77 S cm⁻¹ and an elongation at break of about 32.5%. More interestingly, they exhibit a high EMI shielding effectiveness (SE) of about 62 dB over the X-band frequency range at a film thickness of only 0.15 mm.