Background: The optimal adjuvant strategy for incidentally detected node-positive rectal cancer following curative intent surgery remains uncertain, particularly in early-stage disease where nodal involvement is only identified postoperatively. This systematic review evaluates the survival impact of available adjuvant modalities and highlights prognostic factors that may justify selective use of postoperative radiotherapy (RT). Methods: A systematic search of PubMed, EMBASE, MEDLINE and the Cochrane Library was conducted to August 2023 following PRISMA guidelines. Retrospective studies reporting outcomes in patients with incidental nodal disease who underwent curative rectal resection without neoadjuvant therapy were included. Risk of bias was assessed using the Newcastle–Ottawa Scale. Heterogeneity precluded meta-analysis. Results: Nine studies comprising 5,989 patients were analysed. Adjuvant therapy demonstrated superior outcomes compared with observation alone. Overall survival ranged from 61.3–92% with adjuvant chemotherapy (CT) and 63–93% with chemoradiotherapy (CRT), compared with 42–82.1% without adjuvant treatment. Disease-free survival ranged from 43–90%. Local recurrence (2–9.1%) was consistently lowest with CRT. Importantly, several studies reported comparable survival between CT and CRT in pT1–2N1 disease, suggesting that routine CRT may not be required in all incidental stage IIIA cases. Poorer outcomes were consistently associated with pN2 disease, positive margins, lymphovascular or perineural invasion, high lymph node ratio and low tumour location. Conclusion: Adjuvant CT should be considered the minimum standard of care for incidental node-positive rectal cancer. RT appears most appropriate for patients with specific high-risk features or unfit for CT rather than universally for all stage IIIA patients. These findings support risk-adapted postoperative decision-making within colorectal multidisciplinary teams, and reinforce the need for prospective, stratified adjuvant treatment protocols.