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The global incidence of rectal cancer (RC) is increasing at an alarming rate, with over 50% of cases still diagnosed at advanced stages, despite improvements in early detection. While neoadjuvant (chemo)radiotherapy regimens remain a cornerstone of treatment, up to 20-40% of tumours exhibit or develop resistance to ionising radiation, limiting therapeutic options and leading to disease evolution. This review offers a comprehensive insight into the complex biological and molecular mechanisms underlying RC radioresistance within the fundamental 6Rs of radiobiology: Repair, Reoxygenation, Redistribution, Radiosensitivity, Repopulation, and Reactivation. More specifically, DNA damage repair pathways, cell cycle regulation, hypoxia, transcriptional plasticity regulators such as cancer stem cells, and non-coding RNAs are comprehensively reviewed. Additionally, the critical roles of tumour-infiltrating lymphocytes, cancer-associated macrophages, and inflammatory cancer-associated fibroblasts in radioresistance are also dissected, which collectively shape an immunosuppressive and pro-metastatic niche following radiotherapy. Furthermore, a less explored mechanism, namely the metabolic rewiring of cancer cells after radiotherapy, is proposed as a new 7th R of Radiobiology, namely "Reprogramming", that enables tumour survival and promotes aggressive phenotypes. This new perspective adds a new layer of complexity to the molecular understanding of RC resistance and provides a mechanistic insight currently missing in radiobiology research. Additionally, advanced experimental models, such as spheroids, patient-derived organoids, and animal models, are discussed as valuable platforms for pre-clinical research and therapeutic testing. This review also integrates mechanistic insights with biomarker-guided clinical decision-making to support RC management. By unravelling these multifactorial mechanisms, we highlight opportunities to develop predictive biomarkers and tailored therapeutic strategies to overcome resistance and improve patient outcomes.