Objectives: Nuclear factor κB (NF-κB) is a superfamily of transcription factors that plays a key role in cancer genesis and progression. The present study aimed to explore the exact role of NF-κB/p65 as a marker in breast cancer and its relationship with other prognostic markers, such as tumour grade, tumour size, hormone receptors, and HER-2 expression. Methods: Ninety-nine unselected formalin-fixed paraffin-embedded invasive ductal and lobular tissue sections were evaluated by immunohistochemistry methods to measure NF-κB/p65 expression, ER, PR, HER-2 and Ki-67. The correlation between NF-κB/p65 and clinicopathological parameters was assessed. Results: NF-κB/p65 was only found in the cytoplasm and positively correlated with large tumours (≥2 cm) and high-grade tumours (P< 0.001 and P= 0.018, respectively). Other breast cancer markers, such as histological type (P= 0.766), HER-2 (P= 0.416), PR (P= 0.356), and ER (P= 0.606), had no significant link with NF-B/p65 expression. Furthermore, no significant relation with the Ki-67 marker was detected (P= 0.117). Conclusion: The current study discovered a link between NF-κB/p65 overexpression and both large tumour size and higher grade. This might mean that the expression of NF-κB/p65 is associated with aggressive biological activity in breast cancer and that studying the mechanisms that lead to NF-κB/p65 cytoplasmic accumulation could lead to the discovery of novel therapeutic methods.