Controlling reactive oxygen species (ROS) without metals has been a long-standing challenge. Peracids are uniquely capable of dual ROS pathways—O2 generation under basic conditions and •OH radical production upon thermal activation—but their rapid, uncontrolled decomposition in solution has rendered them practically unusable. We solve this by confining peracid moieties within a crosslinked poly(ethylene glycol) (PEG) nanogel network. Modulating the degree of topological isolation tunes both decomposition kinetics and pathway selectivity. An inherently unstable species becomes a controllable, switchable ROS source. This work establishes metal-free soft nanoparticles as programmable ROS regulators.