Improving crop productivity while minimizing reliance on chemical fertilizers and pesticides is a central challenge in sustainable agriculture. In this context, plant biostimulants such as brown macroalgae and arbuscular mycorrhizal fungi (AMF) have emerged as effective tools to enhance crop performance by stimulating physiological processes, increasing nutrient use efficiency, and improving stress resilience. This study investigated the effects of three brown macroalgae species (Padina pavonica, Fucus vesiculosus, and Cystoseira tamariscifolia) and AMF, applied individually or in combination, on the growth, yield, fruit quality, and antioxidant activity of a traditional yellow tomato (Solanum lycopersicum L.) landrace. A two-year field experiment was conducted using a randomized complete block design, with eight treatments, including a non-treated control. Biostimulants were applied at transplanting and subsequently via root application. Among the treatments, P. pavonica—particularly when combined with AMF—showed the most promising results. This treatment led to the highest fruit yield (57.3 t ha⁻¹), an increase in the fruit number per plant (20.4 fruits/plant), and a greater total biomass (1.21 kg/plant). Fruit quality was also improved, with significant increases in soluble solids content (4.6 °Brix), firmness (1.42 kg/m²), and brightness (L* value of 61.2). Moreover, the P. pavonica + AMF treatment induced the strongest antioxidant response, with elevated levels of CAT (2.45 µmol min⁻¹ mg⁻¹ protein) and GPOX (23.71 µmol min⁻¹ mg⁻¹ protein) activities. These findings underline the synergistic potential of combining macroalgal extracts with mycorrhizal fungi to sustainably enhance both the yield and nutritional value of tomatoes. This approach offers a viable and environmentally friendly alternative for improving horticultural performance in reduced-input systems.