In this paper, we investigate the cooperative localization problem in non-line-of-sight (NLOS) environments using time-of-arrival measurements. We propose a novel robust localization method that is robust against the effect of NLOS errors on the localization performance. The proposed method only requires knowledge of the upper bound on the magnitude of the NLOS errors rather than their statistics, which are more difficult to obtain in practice. Our method is shown to perform better than the existing non-robust method especially in severe NLOS environments.