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TENT4A (PAPD7) is a non-canonical poly(A) polymerase, of which little is known. Here, we show that TENT4A regulates multiple biological pathways and focuses on its multilayer regulation of translesion DNA synthesis (TLS), in which error-prone DNA polymerases bypass unrepaired DNA lesions. We show that TENT4A regulates mRNA stability and/or translation of DNA polymerase η and RAD18 E3 ligase, which guides the polymerase to replication stalling sites and monoubiquitinates PCNA, thereby enabling recruitment of error-prone DNA polymerases to damaged DNA sites. Remarkably, in addition to the effect on RAD18 mRNA stability via controlling its poly(A) tail, TENT4A indirectly regulates RAD18 via the tumor suppressor CYLD and via the long non-coding antisense RNA <i>PAXIP1-AS2</i>, which had no known function. Knocking down the expression of <i>TENT4A</i> or <i>CYLD</i>, or overexpression of <i>PAXIP1-AS2</i> led each to reduced amounts of the RAD18 protein and DNA polymerase η, leading to reduced TLS, highlighting <i>PAXIP1-AS2</i> as a new TLS regulator. Bioinformatics analysis revealed that TLS error-prone DNA polymerase genes and their <i>TENT4A</i>-related regulators are frequently mutated in endometrial cancer genomes, suggesting that TLS is dysregulated in this cancer.