<p>Cadmium (Cd) is a major environmental pollutant that is toxic to plants, animals, and humans. While many studies have demonstrated the ability of silicon (Si) to alleviate abiotic stress in plants, its role and potential mitigating mechanisms in pseudocereals are still underexplored. The aim of this study was to investigate the effect of exogenous Si (1 and 2.5 mM) on Cd (100 µM) toxicity in grain amaranth (<i>Amaranthus cruentus</i> L.) grown in hydroponics for 14 days. We evaluated various physiological traits such as plant growth, morphological and anatomical parameters of roots, leaf stomata and epidermal cell density, as well as nutrient uptake, expression of stress-related genes, antioxidant enzyme activities and photosynthetic pigment contents. The results showed that the addition of Si to Cd treatment increased all measured growth parameters and restored root tissue arrangement, including xylem area, compared to Cd-treated plants. We found that Cd accumulation significantly decreased under Si supplementation in a Si concentration-dependent manner, translocation of toxic Cd ions to the aerial parts was low, and Cd ions remained mainly stored in the roots. The application of Si had no obvious effect on the uptake of essential micro- and macroelements in amaranth roots, which was reduced by Cd. The effect of Si was more pronounced at the level of antioxidants and gene expression. Overall, our results showed that Si, at a concentration of 1 mM, effectively attenuated the negative effects of Cd on the vegetative growth of amaranth.</p>