light by the devices. Currently indium tin oxide (ITO) coated on rigid glass is used as the standard transparent electrode. However, ITO has several problems including the scarcity of indium on earth and mechanical brittleness, [2][3][4] which not only increase the cost of the electronic devices but also limit its application in flexible devices. Additionally, ITO has inherent problems such as the release of oxygen and indium into the organic layer, poor transparency in blue region and high processing temperature. [5] Therefore, new transparent conductive materials with high mechanical flexibility and low cost are urgently needed to replace ITO. To respond to this need, conducting polymers, [6][7][8][9] carbon-based nanomaterials, [10][11][12][13] metallic nanostructures, [14][15][16][17] and multilayer thin films [18][19][20][21] have been explored as transparent electrode. Among them, poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS, chemical structure shown in Scheme 1) has received wide attention due to its high optical transmittance in visible range, excellent mechanical flexibility, superior thermal stability, and compatibility with various printing techniques. [22][23][24][25] However, pristine PEDOT:PSS film exhibits low conductivity of less than 1 S cm −1 , which hinders its use as a transparent electrode. Readily obtained highly conductive, transparent, and flexible poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films is urgently needed in printing of flexible transparent electrodes. A simple and facile method to enhance the electrical conductivity of PEDOT:PSS films is reported. The conductivity is increased by four orders of magnitude after adding solid chloroplatinic acid (H 2 PtCl 6 ) into the pristine PEDOT:PSS solution. The H 2 PtCl 6 -doped PEDOT:PSS film exhibits a sheet resistance of 44 ± 5 Ω -1 and a transmittance of 84 ± 1% at 550 nm, corresponding to a figure of merit of 47 ± 4. Comparative study shows addition of solid acid like H 2 PtCl 6 is more effective in conductivity enhancement than addition of polar organic solvents, such as dimethyl sulfoxide or ethylene glycol. The mechanism for the conductivity enhancement is attributed to both in situ doping and phase separation of PEDOT:PSS. PEDOT is oxidized and doped by Pt 4+ of H 2 PtCl 6 , which is reduced simultaneously to Pt 2+ .