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Accumulated studies have revealed that liquid-liquid phase separation (LLPS) plays a significant role in regulating DNA repair. Our previous studies have identified a list of DNA repair-related genes that may undergo LLPS, including REXO4. In this study, we aim to elucidate whether REXO4 undergoes LLPS and its role in DNA damage response and tumor radioresistance. To identify the LLPS characteristics of REXO4 in vivo , puncta formation assay and fluorescence recovery after photobleaching (FRAP) were performed in HEK 293T cells overexpressing REXO4-mEGFP. For in vitro droplet formation assay, recombinant REXO4-mEGFP proteins were expressed in Escherichia coli and purified with GST-tag resin. Colony formation, mouse xenograft model and γ-H2A.X detection assay were performed to examine the impact on DNA repair and radiosensitivity. In vitro droplets pelleting assay followed by mass spectrometry (MS) and Duolink proximity ligation assay were used to explore the recruitment of PARP1 at DNA double-strand breaks (DSBs). REXO4-GFP was ectopically expressed in HEK293T cells and was observed to form discrete spherical puncta. FRAP analysis showed that REXO4-GFP puncta recovered fluorescence on a time scale of seconds. The 1,6-Hexanediol treatment dissolved the puncta rapidly. Purified recombinant REXO4-GFP protein formed droplets in a concentration-dependent manner. The adjacent droplets were observed fusing into a larger one. These results suggested that REXO4 underwent LLPS both in vivo and in vitro. Meanwhile, IDR (1-232 aa) was identified as the required domain for REXO4 phase separation. It was reported that the amino acid bias in IDR may be essential for LLPS. In line with this, we mutated all the highly repeated lysine in IDR and found that the lysine bias was vital for REXO4 condensation. Further, we performed comet assay and γ-H2A.X detection after irradiation and found that REXO4-KO cells exhibited delayed γ-H2A.X clearance and longer comet tails, indicating that REXO4 enhanced IR-induced DNA repair. Furthermore, droplet pelleting assay coupled with MS experiment revealed that REXO4 condensates recruited PARP1, which catalyzes Poly(ADP-ribosyl)ation of the target proteins at DSBs to promote DSBs repair. CoIP assay showed that REXO4 interacted with PARP1, while the interaction between LLPS-deficient REXO4 and PARP1 was attenuated. Further, we demonstrated that re-expressing wildtype REXO4 restored the irradiation-induced Poly(ADP-ribosyl)ation in the REXO4-depleted cells, whereas re-expressing the LLPS-deficient REXO4 had no effect. Analysis of REXO4 expression in clinical samples showed that REXO4 was upregulated in colorectal tumor tissues and its high expression was related to tumor radioresistance. REXO4 underwent LLPS in vivo and in vitro , which enhanced the recruitment of PAPR1 at DSBs and the IR-induced Poly(ADP-ribosyl)ation, thereby promoting DSBs repair and tumor radioresistance. [ABSTRACT FROM AUTHOR]