Although it was originally thought of as a passive way to block the nuclear function of p53, accumulating evidence suggests that cytoplasmic localization of p53 plays an active role in p53-mediated functions such as apoptosis and autophagy. Previous studies by us and others demonstrated that Mdm2-mediated p53 ubiquitination induces both degradation and cytoplasmic localization. Here we describe MSL2, a novel E3 ligase for p53 that promotes ubiquitin-dependent cytoplasmic p53 localization. Unlike Mdm2 or most other p53 E3 ligases, MSL2-mediated p53 ubiquitination does not affect the stability of p53. Moreover, the MSL2-mediated effect on p53 is Mdm2-independent. Thus, our study identifies an important ubiquitin-ligase for modulating p53 subcellular localization.