In this study, we report on the fabrication of a highly conductive, stretchable, transparent electrode based on modified poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with triblock copolymer (PEO20–PPO70–PEO20, Pluronic® P123) and post-treatment via sulfuric acid. Sulfuric acid post treatment is well known to increase the electrical conductivity of PEDOT:PSS via crystallization of PEDOT, conductive part of PEDOT:PSS. Since the sulfuric acid treated PEDOT:PSS film is not stretchable, however, we introduce P123 as a secondary dopant and plasticizer to enhance the stretchability via crosslinking as well as the electrical conductivity of PEDOT:PSS. The fabricated electrode shows high transparency of 89%, high electrical conductivity of ~1,700 S/cm, and minimal change in resistance of ~4% during repetitive stretch–release cycles by 40% tensile strain. By utilizing the high transparency and low resistance change with stretching of the electrode, a stretchable transparent touch sensor array with light emitting diodes is fabricated and it exhibits mechanical stability upon stretching by 30%. This work demonstrates the facile chemical modification to develop PEDOT:PSS based stretchable electrode with high transparency and high electrical conductivity.