Magnesium oxide (MgO) is recognised as exhibiting a contact‐based antibacterial activity. However, a comprehensive study of the impact of atomic‐scale surface features on MgO's antibacterial activity is lacking. In this study, the nature and abundance of the native surface defects on different MgO powders are thoroughly investigated. Their impacts on the hydrolysis kinetics, antibacterial activity against Escherichia coli (ATCC 47076), Staphylococcus epidermidis and Pseudomonas aeruginosa and the reactive oxygen species (ROS) generation potential are determined and explained. It is shown that a reduction in the abundance of low‐coordinated oxygen atoms on the surface of the MgO improves its resistance to both hydrolysis and antibacterial activity. The ROS generation potential, determined in‐situ using a fluorescence microplate assay and electron paramagnetic resonance spectroscopy, is not an inherent property of the studied MgO, rather it is a side product of hydrolysis (only for the most highly defected MgO particles) and/or a consequence of the MgO/bacteria interaction. The evaluation of the mutual correlations of the hydrolysis, the antibacterial activity and the ROS generation, with their origin in the surface defects' peculiarities, led to the conclusion that the acid/base reaction between the MgO surface and the bacterial wall contributes considerably to the MgO's antibacterial activity. The manipulation of native defects at the surface of MgO is an effective tool to regulate surface hydrolysis and antibacterial activity. The processing of an oxygen‐deficient MgO surface leads to optimal bactericidal activity due to slower hydrolysis, while reactive oxygen species are generated only as a side‐product of chemical MgO/bacteria interactions.