Breast cancer is the most common malignancy among women worldwide. Kynureninase (KYNU) located in 2q22.2, which was associated with tryptophan utilization and metabolic diseases including cardiac, renal and limb defects syndrome 2. However, the role of KYNU in breast cancer (BC) development remains unclear. The expression of KYNU was examined by immunohistochemistry (IHC) in 137 primary BC tissues, and the correlation of KYNU expression with clinical pathological characteristics and the biomarkers (ER, PR, HER2, E‐cad and Ki‐67) was analysed. The role of KYNU in cancer cell proliferation, tumour growth and development was evaluated by MTT assay, soft agar colony formation assay and xenograft mouse models. Among 137 primary BC tissues, 46.7% (64/137) had high KYNU expression (IHC scores >4) while 53.3% (73/137) had low KYNU expression (IHC scores ≤4). The expression of KYNU was positively correlated with the expressions of ER ( P = .002), PR ( P = .007) and E‐cad ( P = .03), while negatively associated with tumour grade ( P = .008), tumour stage ( P < .001) and the expressions of HER2 ( P = .04) and Ki‐67 ( P = .019). Overexpression of KYNU significantly inhibited cell proliferation in cell culture, colony formation in soft agar and xenograft BC development in NOD/SCID mice. Kynureninase suppresses BC cell proliferation, tumour growth and development. Kynureninase may function as a tumour suppressor in BC.