Tuta absoluta is a major pest, damaging tomato crops in many countries. Spinetoram, a novel insecticide, is increasingly used for the management of various insect pests. However, limited information is available on its lethal effects on T. absoluta. Therefore, this study was carried out to evaluate the toxicity of spinetoram concentrations in T. absoluta by exposing third-instar larvae to this insecticide through leaf-dipping bioassays under laboratory conditions. The results showed that sublethal spinetoram concentrations (LC₁₀ and LC₂₀) extended the developmental duration of the larval and pupal stage, as well as the oviposition period, and decreased the pupal weight, pupation and eclosion rates, fecundity, sex ratio, and egg-hatching rate compared to the control group. LC₁₀ and LC₂₀ treatments up-regulated the activity of superoxide dismutase and catalase antioxidant enzymes in T. absoluta larvae by 51.14–88.72% and 39.34–87.76% at 12 h, 62.71–96.68% and 37.38–39.92% at 24 h, and 41.78–53.34% and 19.44–48.39% at 48 h, respectively, compared to the control. Conversely, the LC₁₀ and LC₂₀ treatments inhibited the activities of detoxifying enzymes, i.e., carboxylesterase (14.72–34.95%), acetylcholinesterase (9.02–48.86%), glutathione S-transferase (19.40–43.19%), and mixed-function oxidase (18.18–25.93%). These findings indicate that lethal concentrations of spinetoram influence the population dynamics of T. absoluta by reducing their survival and developmental potential and inhibiting the activity of detoxifying enzymes.