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This is an accepted article with a DOI pre-assigned that is not yet published.Wild leafy vegetables are increasingly recognised both for their contribution to food security and as sources of nutraceuticals due to their accumulation of bioactive, redox-active phytochemicals. In this study, we profiled the phytochemical composition and in vitro antioxidant potential of methanolic leaf extracts of wild Alternanthera sessilis collected from the Forestry Research Institute of Nigeria (Ibadan). Leaves were air-dried, milled and extracted in methanol. Qualitative screening employed standard spot tests. Gravimetric assays quantified total alkaloids, saponins and terpenoids. Spectrophotometric methods determined tannin (tannic-acid equivalents), total flavonoids (quercetin equivalents) and total phenolics (gallic-acid equivalents). Antioxidant activity was assessed by DPPH radical scavenging, FRAP, and phosphomolybdenum (TAC) reducing-power assays. Qualitative screening detected alkaloids, flavonoids, terpenoids, tannins, saponins, steroids, phenols and anthraquinones. Quantitative data indicated alkaloids > terpenoids > saponins (≈1.6%, 1.1% and 0.8% w/w, respectively). Spectrophotometric assays yielded mean (±SD) values of tannin 629.69 ± 363.60 mg TAE/g, TFC 315.13 ± 44.56 mg QE/g and TPC 755.31 ± 498.80 mg GAE/g. DPPH IC₅₀ values were 229.56 µg·mL⁻¹ (extract) and 23.21 µg·mL⁻¹ (ascorbic acid); FRAP assay produced ascorbic-acid equivalents (AAE) (≈12.04–25.42 µg AAE per mg extract) at higher assay concentrations (60-100 µg·mL⁻¹), while the TAC AAE values of 116.57–580.33 mg AAE/g across 20–100 µg·mL⁻¹. Overall, these results indicate that A. sessilis leaves constitute a significant reservoir of redox-active compounds. These findings justify further chromatographic fractionation and compound-level analyses to isolate and characterise the major contributors to its antioxidant effects and assess their potential nutraceutical applications.