Sepsis, a systemic inflammatory response syndrome, causes severe immune dysfunction and is associated with high mortality because of the lack of effective clinical interventions. To address the pathogenesis of sepsis, such as bacterial infection and the exacerbation of inflammation and oxidative stress, an acidity-activated polylysine (PLL)-based copolymer self-assembly (PPDD) was developed. This material was synthesized by conjugating polyethylene glycol-modified PLL (PEG-PLL) with 2,7-dichlorofluorescein diacetate (DCFH-DA). PPDD, with its PLL-derived antibacterial and antioxidant properties, can scavenge reactive oxygen species (ROS), mitigate inflammation and eliminate bacteria. These combined actions help alleviate the symptoms of sepsis and improve survival rates. In vitro and in vivo experiments confirmed that this approach can rapidly neutralize ROS, significantly reduce pro-inflammatory cytokine cascades and effectively clear bacteria, thereby improving physiological stability and survival rates. Notably, Day-14 survival reached 80% in the PPDD-treated group compared with 20% in septic controls. More significantly, when the PPDD copolymer self-assembles into the acidic sepsis microenvironment, it disassembles and reconfigures from a spherical to an ellipsoidal structure. This acidity-activated structural transformation exposes more bioactive components for ROS scavenging, which is beneficial for removing oxidative stress, killing bacteria, reducing inflammation and alleviating sepsis. Following PPDD administration, systemic levels of TNF-α, IL-6, IL-10 and CRP were reduced by 38.1%, 46.0%, 76.7% and 32.9%, respectively, confirming its robust anti-inflammatory effect. Additionally, the conjugated DCFH-DA, a cell-permeable fluorescent probe, enables monitoring of oxidative stress and tracing the evolution of sepsis, especially after treatment. A comprehensive biosafety assay revealed no detectable hemolysis or organ toxicity, substantiating the translational potential of this platform. Our biocompatible and acidic sepsis environment-responsive PPDD paves a solid foundation for the clinical diagnosis and treatment of sepsis.© The Author(s) 2025. Published by Oxford University Press.