Single atom nanozymes are the artificial enzymes with enzyme-like activity, which have attracted a great deal attention in recent years due to their unique merits such as remarkable stability, excellent atom utilization and low cost. Herein, a convenient and sensitive colorimetric strategy was developed for the sensing of galactose based on Fe–N–C single-atom nanozyme (Fe-SAzyme). The Fe-SAzyme was prepared through “isolation-pyrolysis” method that exhibited intrinsic peroxidase mimicking activity, which can quickly catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to produce blue-colored oxTMB in the presence of hydrogen peroxide (H2O2). Galactose can be oxidized by galactose oxidase (Gal Ox) to generate H2O2, and Fe-SAzyme can be utilized for quantitative colorimetric detection of galactose. A good linearity between absorbance and the galactose concentration in the range of 50–500 μM was obtained with a detection limit of (LOD) 10 μM. The Fe-SAzyme based colorimetric strategy offered a rapid, convenient and economic way for galactose quantification detection, which could be used as an alternative method for galactosemia diagnosis. Display omitted •The Fe-SAzyme was synthesized by low-cost precursors and simple procedure.•The Fe-SAzyme displayed high stability and excellent peroxidase-like activity.•A convenient and economic colorimetric detection method for H2O2 and galactose has been successfully established.•This proposed strategy exhibited satisfactory performance for galactose assay in human serum samples.