Dielectric elastomers are composite thin film structures composed of a dielectric polymer between compliant electrodes. Previous hyperelastic models have not modeled the constraint effects of compliant electrodes on the lateral contraction of the dielectric material as it stretches, and are therefore unable to fully describe the electromechanical behavior of the dielectric elastomer or provide a means to understand the constraint effects. An empirical boundary coefficient is introduced to model these constraint effects on the lateral boundaries of the material under uniaxial tension. Employing an averaged stretch ratio concept, it is shown that this coefficient can be obtained from experimentally measurable geometric variables. Values for the boundary coefficients of sample dielectric elastomer films were obtained from experiments performed on a uniaxial test stand. Incorporating the boundary coefficient into the model formulation, a specific hyperelastic stress–strain relation is derived to describe the electromechanical behavior of dielectric elastomers under combined uniaxial tension and electrical loading. Comparison of the experimental and predicted values of the induced force in the axial direction due to the Maxwell stress based on the uniaxial model shows favorable agreement.