<p>The interactive effects of multi-walled carbon nanotubes (MWCNTs) and salinity stress on tomato (<i>Solanum lycopersicum</i> L.) callus, focusing on callus induction, growth, redox homeostasis, nutrient uptake, and secondary metabolism, were investigated. Using a two-step <i>in vitro</i> system, callus was induced on full-strength MS medium supplemented with 0.5 mg L⁻¹ naphthaleneacetic acid (NAA) and 2.5 mg L⁻¹ 6-benzylaminopurine (BAP), followed by a 21-day exposure to NaCl (100.0 mM) and MWCNTs (0.0, 100.0, and 150.0 mg L⁻¹). Treatment with 100.0 mg L⁻¹ multi-walled carbon nanotubes (MWCNTs) significantly improved biomass production, particularly under salt stress, and also enhanced uptake of Ca²⁺, Fe²⁺, and PO₄³⁻ and stimulated antioxidant mechanisms. Under salinity, 100.0 mg L⁻¹ MWCNTs reduced MDA and H₂O₂ levels by 62.0% and 20.0% (<i>P</i> ≤ 0.05), respectively, compared to NaCl alone. Meanwhile, both enzymatic and non-enzymatic antioxidant activities were enhanced. Exposure to 100.0&#xa0;mM NaCl alone elevated phenolics (8.6%), anthocyanins (34.0%), and lycopene (33.5%) relative to the control. However, co-application with 100.0&#xa0;mg L⁻¹ MWCNTs further enhanced these responses, reaching 39.0%, 84.0%, and 55.6%, respectively. PAL activity exhibited a similar pattern, with pronounced induction under combined stress. MWCNTs failed to confer further improvement above the 100.0&#xa0;mg L⁻¹ dose. Collectively, these results identify 100.0&#xa0;mg L⁻¹ as the optimal dose, and demonstrate the dose-dependent potential of MWCNTs to modulate stress responses and act as effective elicitors in plant tissue culture and abiotic stress management.</p>