Microgreens are increasingly popular due to consumer demand for healthier foods and a shift toward sustainable agriculture, especially in saline-affected areas. Humic acids (HAs) have shown potential in enhancing nutrient uptake and improving plant resilience under stress. This study evaluated the effects of salinity stress (0–80 mM sodium chloride, NaCl) and HA (0–5 g/L) on the growth and phytochemical composition of basil (Ocimum basilicum L.) microgreens. Seeds were sown in vermiculite-filled trays and grown under greenhouse conditions, with a 6-day dark germination period. Salinity and HA treatments were co-applied via irrigation from sowing until harvest (15 days after sowing). Growth parameters (height and colour) were recorded. Chlorophylls, organic acids, tocopherols, fatty acids, and minerals were analyzed using chromatographic and spectroscopic techniques. Total phenolic content (TPC) and total flavonoid content (TFC) were determined colorimetrically, and antioxidant activity was assessed using the thiobarbituric acid reactive substances (TBARS) and oxidative hemolysis inhibition assay (OxHLIA). Eighteen fatty acids, three tocopherols, and five organic acids were identified. Salinity and HA significantly affected most chemical parameters (p < 0.05), though plant height and colour were unaffected. Microgreens treated with 40 mM NaCl and 3.5 g/L HA accumulated the highest mineral content (2.86 g/100 g fresh weight). The highest TFC (0.4 mg of quercetin equivalents/g extract) was observed with 40 mM NaCl and 1.5 g/L HA, while the highest TPC (7.3 mg of gallic acid equivalents/g extract) occurred at 0 mM NaCl and 5.0 g/L HA. The strongest antioxidant responses were recorded in samples treated with 40 mM NaCl plus 5.0 g/L HA (OxHLIA, IC₅₀ = 28.8 µg/mL) and 0 mM NaCl plus 2.5 g/L HA (TBARS, EC₅₀ = 45.2 µg/mL). HA likely promoted osmotic adjustment, enhancing nutrient uptake and antioxidant defense, supporting better phytochemical profiles under stress. Its effects were most notable under moderate salinity.