In this work, we report the discovery of a carboxyl-containing iron catalyst 1 (Fe^II-DCBPY, DCBPY = 2,2′-bipyridine-4,4′-dicarboxylic acid), which could activate the C–H bonds of cycloalkanes with high secondary (2°) C–H bond selectivity. A turnover number (TN) of 11.8 and a 30% yield (based on the H₂O₂ oxidant) were achieved during the catalytic oxidation of cyclohexane by 1 under irradiation with visible light. For the transformation of cycloalkanes and bicyclic decalins with both 2° and tertiary (3°) C–H bonds, 1 always preferred to oxidise the 2° C–H bonds to the corresponding ketone and alcohol products; the 2°/3° ratio ranged between 78/22 and >99/1 across 7 examples. ¹⁸O isotope labelling experiments, ESR experiments, a PPh₃ method and the catalase method were used to characterize the reaction process during the oxidation. The success of 1 showed that, in addition to using a bulky catalyst, high 2° C–H bond selectivity could also be achieved using a less bulky molecular iron complex as the catalyst.