Mitochondrial reactive oxygen species (mROS) are generated as byproducts of mitochondrial oxidative phosphorylation. Changes in mROS levels are involved in tumorigenesis through their effects on cancer genome instability, sustained cancer cell survival, metabolic reprogramming, and tumor metastasis. Recent advances in nanotechnology offer a promising approach for precise regulation of mROS by either enhancing or depleting mROS generation. This review examines the association between dysregulated mROS levels and key cancer hallmarks. We also discuss the potential applications of mROS-targeted nanoparticles that artificially manipulate ROS levels in the mitochondria to achieve precise delivery of antitumor drugs.