Background: Cancer cells develop multidrug resistance after receiving fractionated ionizing radiation. However, the mechanisms underlying this phenomenon remain unknown. This study aimed to investigate the role of platelet/endothelial cell adhesion molecule 1 (PECAM-1), which was induced by ionizing radiation, in overcoming cisplatin resistance of nasopharyngeal carcinoma (NPC) cells. Materials and Methods: Human NPC cell line CNE1 was subjected to fractionated ionizing radiation to obtain a subline with the phenotype of multidrug resistance (designated as CNE1/R). PECAM-1 gene expression in CNE1/R cells was knocked down by stable transfection of pSilencer plasmid carrying specific small hairpin RNA. The transcripts of PECAM-1 and multidrug resistance gene 1 (MDR1) were analyzed by reverse transcription–polymerase chain reaction, and their encoding proteins were detected by Western blot analysis. The in-vitro viability of tumor cells was examined with MTT assay and flow cytometry analysis. The tumor growth in xenograft mice was determined by measuring tumor weights. Results: The transcript and protein levels of PECAM-1 and MDR1 were concomitantly upregulated in CNE1 cells subjected to ionizing radiation. The inhibition of PECAM-1 expression with small hairpin RNA reduced the levels of MDR1 transcript and its encoding protein, P- glycoprotein. Furthermore, targeting PECAM-1 not only enhanced the sensitivity of irradiated CNE1 cells to cisplatin-mediated cell cytotoxicity in-vitro but also resulted in tumor regression in-vivo. Conclusions: An increased PECAM-1 level in CNE1 cancer cells subjected to ionizing radiation contributed to cisplatin resistance via the upregulation of MDR1 expression. Thus, targeting PECAM-1 might help overcome drug resistance induced by ionizing radiation in CNE1 NPC cells.