Radiation combined with chemotherapy (neo-CRT) is increasingly the standard of care for the treatment of esophageal cancer, either as neoadjuvant therapy in multimodal protocols or as primary therapy. Unfortunately, ~60% of patients demonstrate little or no response to neo-CRT. Accordingly, understanding the molecular mechanisms of resistance to therapy may underpin significant advances through the identification of nonresponders either before or early in treatment. We previously identified the RNPC1 gene, which is important in stabilizing p21, as being upregulated in the tumors of esophageal cancer patients who had a poor response to neo-CRT. We hypothesize that RNPC1 contributes to resistance to radiation therapy through a p21-mediated cell cycle accumulation/arrest mechanism. Analysis revealed that p53 and RNPC1 expression were highest in the JH-EsoAd1 cell line and lowest in OE19 cells. This was associated with accumulation of cells in G₀/G₁. p21 expression, which was highest in OE19 cells and lowest in OE33 cells, was associated with relative intrinsic sensitivity to radiation. OE33 cells were transfected with a plasmid (pCMV6-AC-GFP) encoding a C-terminal GFP-tagged RNPC1, and overexpression was confirmed by qPCR and fluorescence microscopy. Overexpression of RNPC1-GFP resulted in significantly increased levels of the p21 transcript and protein through a direct physical interaction between the RNPC1 protein and the p21 transcript. Furthermore, RNPC1 overexpression led to significant G₀/G₁ cell cycle accumulation and significantly enhanced cellular resistance to radiation. We conclude that RNPC1 contributes to tumor resistance to radiotherapy, which likely occurs through a p21-mediated G₀/G₁ accumulation mechanism. Therefore, RNPC1 may represent a potential therapeutic target for enhancing tumor sensitivity to radiation.