Translesion DNA synthesis (TLS) is a mode of DNA damage tolerance which plays an important role in genome mutagenesis and chromatin integrity maintenance. Proliferating cell nuclear antigen (PCNA) monoubiquitylation is one of the key factors for TLS pathway choice. So far, it remains unclear how the TLS pathway is elaborately regulated. Here, we report that TLS polymerase REV1 can promote PCNA monoubiquitylation after UV radiation. Further studies revealed that this stimulatory effect is mediated through the enhanced interaction between REV1 and ubiquitylated RAD18, which facilitates the release of nonubiquitylated RAD18 from ubiquitylated RAD18 trapping, after which RAD18 is recruited to chromatin for its TLS function. Furthermore, we found that this stimulatory effect could also be detected after exposure to hydroxyurea or mitomycin C, but not methyl methanesulfonate (MMS), which is in line with the fact that ubiquitylated RAD18 could not be detected after exposure to MMS.