A three-parameter elastic-plastic strain hardening theory for prediction of biaxial stress-strain response of concrete is developed in terms of accumulated equivalent plastic strains in tension and compression. The hardening rule permits individual variation of the current yield value in compression and the current yield values in tension in two principal directions. Hardening functions are determined from tensile and compressive uniaxial tests. The theory is intended for use in the analysis of prestressed thin-shell containment structures. Predictions of stress-strain response are compared with experimental data of material response available in the literature. The technique is then used to predict the response of two large-scale prestressed concrete wall segment biaxial tension tests and the results are compared to the experimental data. The results indicate that the theory can form a basis for practical inelastic analysis of prestressed concrete thin-shell tension structures.