The development of stretchable electronic devices that are soft and conformable has relied heavily on a single material-polydimethylsiloxane-as the elastomeric substrate. Although polydimethylsiloxane has a number of advantageous characteristics, its high gas permeability is detrimental to stretchable devices that use materials sensitive to oxygen and water vapor, such as organic semiconductors and oxidizable metals. Failing to protect these materials from atmosphere-induced decomposition leads to premature device failure; therefore, it is imperative to develop elastomers with gas barrier properties that enable stretchable electronics with practical lifetimes. Here, butyl rubber-a material with an intrinsically low gas permeability traditionally used in the innerliners of tires to maintain air pressure-is reinvented for stretchable electronics. This new material is smooth and optically transparent, possesses the low gas permeability typical of butyl rubber, and vastly outperforms polydimethylsiloxane as an encapsulating barrier to prevent the atmospheric degradation of sensitive electronic materials and the premature failure of functioning organic devices. The merits of transparent butyl rubber presented here position this material as an important counterpart to polydimethylsiloxane that will enable future generation stretchable electronics.