We develop a variational retinex algorithm for enhancing single underwater image with multiorder gradient priors of reflectance and illumination. First, a simple yet effective color correction approach is used to remove color casts and recover naturalness. Then, a variational retinex model for enhancing the color-corrected underwater image is established by imposing multiorder gradient priors of reflectance and illumination. According to structural sparsity difference between illumination and reflectance, the l1 norm is accurately adopted to model piecewise and piecewise linear approximations on the reflectance, while the l2 norm is appropriately employed to enforce spatial smoothness and spatial linear smoothness on the illumination. Next, a complex underwater image enhancement issue is turned into simple denoising subproblems, which can be addressed by an efficient optimization algorithm that is fast performed on pixel-wise operations without requiring additional prior knowledge about underwater imaging conditions. Final experiments demonstrate that the proposed method yields better results of qualitative and quantitative assessments than several traditional and leading underwater image enhancement approaches.