A new control strategy is required to face the resistance of the cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae),to chemical insecticides, and to decrease the pollution of the environment. Silver nanoparticles (AgNPs) were used to overcome these problems and to control S. littoralis at the immature stage. AgNPs were prepared by the reduction method. The prepared AgNPs were sphere-shaped with an average diameter of 20.73 nm. Several concentrations of AgNPs (50, 100, 200, 400, and 800 ppm) were used as insecticides against the third instar larvae of S. littoralis under laboratory conditions. The feeding treatments and adsorption via the integument produced malformations and morphological changes in the treated S. littoralis larvae. The larvicidal effect of AgNPs recorded LC₅₀ equal to 224.8 ppm after 14 days of the application. Weight loss in the larvae and the pupae after treatment was significant with averages 47.13 and 60.41 percent, respectively. The treatment produced malformations in the shape and size of larvae and pupae. Some metal-detoxifying enzymes were significantly affected due to the treatment. There were quantitative and qualitative changes in the protein contents, in addition to a significant reduction in the total lipids and total carbohydrates of the treated larvae compared with the untreated larvae. The application of AgNPs against S. littoralis larva, as demonstrated by our findings, paves the way for new pest control options.