It is of significance to develop stretchable conductors for flexible electronics. Although intrinsically conducting polymers like poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) can exhibit high conductivity, they have limited mechanical stretchability because of the rigid conjugated backbone and strong interchain interaction. Here, we report the significant enhancement in the mechanical stretchability of PEDOT:PSS films by adding a compound of two or more hydroxyl groups like glycerol, malic acid, 1,2,6-hexanetriol, or triethylene glycol. The elongation at break can be enhanced from <10% of pristine PEDOT:PSS films to >50%. The enhancement in the mechanical stretchability is less significant when other compounds with only one hydroxyl or no hydroxyl group are used. The effect of the compounds with multiple hydroxyl groups on the stretchability of PEDOT:PSS is related to the Hansen solubility parameter (HSP) delta(h). A compound with a higher delta(h) value can give rise to a more significant plasticization of PEDOT:PSS. The mechanism is attributed to the destruction of hydrogen bonds among the chains of poly(styrenesulfonic acid) (PSSH) by the compounds of multiple hydroxyl groups. This effectively lowers the interchain interaction among the PSSH and thus increases the mechanical stretchability of PEDOT:PSS. Simultaneously, these compounds can induce secondary doping to saliently enhance the conductivity of PEDOT:PSS films. The highly stretchable and highly conductive PEDOT:PSS films can be used as transparent stretchable conductors.