When untreated wastewater is used for the irrigation of crops, dissolved heavy metals may impose negative impacts on plant growth and pose health risks. However, silicon nutrition may improve plant tolerance to metal toxicity through external and internal plant mechanisms. This work aims at investigating the effects of silicon on copper, nickel, manganese, cadmium and zinc detoxification in tomato plants. The plants were grown for 50 days by irrigating with tap water and wastewater and having silicon added at 25, 50 and 75 mg kg–1 soil. Results revealed that wastewater irrigation caused an increase of 277–480% in copper, 178–233% nickel, 355–680% manganese, 500–900% cadmium and 117–337% zinc in tomato plants compared to tap water irrigation. The root:stem metal ratios showed that a major portion of absorbed metals was translocated to aerial plant parts when wastewater was applied without silicon. However, silicon supplementation precipitated the metals in soil and influenced their uptake and partitioning within the plant body. The shoot dry matter of tomato plants was negatively correlated with wastewater-induced stem metal concentrations. This study suggests that silicon-assisted metal tolerance of tomato plants was attributed to metal precipitation in soil, complexation in roots and, hence, reduction in their translocation to stems and leaves.