A novel, fast, stable, and cost-effective colorimetric hydrogen peroxide (H2O2) sensor was prepared by the incorporation of silver nanoparticles (AgNPs) in chitosan-co-acrylic acid (CS-co-AAc) microgel. AgNPs were synthesized by green method by using Euphorbia Maculate (EM) leaf extract. Chitosan-co-acrylic acid/silver nanaoparticle (CS-co-AAc/AgNPs) hybrid microgel can be characterized by various analytical techniques. CS-co-AAc/AgNPs hybrid microgel exhibited UV-visible absorption band 436 nm. X-ray diffraction spectrum reveals that CS-co-AAc/AgNPs hybrid microgel are amorphous in nature and the crystalline nature of synthesized AgNPs. The hybrid microgels are highly selective towards H2O2 and the absorption band intensity is proportional to the concentration of H2O2 in solution. Over the concentration range of 0–50 nM, the intensity of the absorption peak decreases linearly,and the corresponding correlation coefficient is R2 -0.9063. The limit of detection is determined to be 3.2 nM. Antibacterial activities of the hybrid microgel were also observed against aquatic bacteria such as Escherichia coli (E.coli) and Staphylococcus aureus (S.aureus) isolated from river water samples taken from Kerala. Practical applicability of the hybrid microgel for the sensing of H2O2 in real milk samples was also investigated. AgNPs themselves can show similar behaviors, but they are destabilized because it undergo rapidly agglomerate. This hybrid microgel system offers numerous advantages, such as sample processing and extreme stability. This technique is ideal for assessing diverse biological and environmental materials since it is easy to use, inexpensive, and adaptable to complex matrices.Graphical Abstract