<p>Soil acidification driven by anthropogenic pressures and insufficient organic amendments critically undermines soil fertility, microbial diversity, and sustainable crop productivity. This study assessed the restorative potential of <i>Sargassum wightii</i> macroalgal extract (SWE) on acid-degraded soils and its ecological safety toward the earthworm <i>Eudrilus eugeniae</i>. The collected soil exhibited pronounced acidity (pH 4.28) with elevated concentrations of exchangeable Al³⁺ and H⁺ ions (2.4 and 0.16 cmol kg⁻¹), coupled with low potassium (107.2 mg kg^-1), organic carbon (1.08%), and nitrogen (0.072 g kg⁻¹). Following 60 days of SWE amendment, soil pH increased to 6.3, while Al³⁺ and H⁺ ions declined markedly (2.02 and 0.314 cmol kg⁻¹, respectively). Enhanced nutrient status was reflected by higher K (288.6 mg kg^-1), N (1.29 g kg⁻¹), and SOC (2.8%) levels. Although soil texture remained sandy loam, SWE improved silt and clay fractions, aggregate stability, and water-retention potential. Microbial abundance (bacteria, fungi, actinomycetes) and enzymatic activities phenol oxidase, peroxidase, urease, dehydrogenase, catalase, and β-glucosidase were significantly stimulated, indicating reactivation of biological functionality. Ecotoxicological evaluation confirmed the non-toxic nature of SWE, as earthworms in treated soils exhibited complete survival compared with 56% mortality under untreated acidic conditions. These findings underscore the dual role of <i>S. wightii</i> as a natural bio stimulant and bio fertilizer, capable of simultaneously neutralizing soil acidity, enhancing microbial metabolism, and promoting eco-safe soil restoration within sustainable agricultural frameworks.</p>