Lead is a toxic element that accumulates in agricultural soils through various anthropogenic sources. It inhibits the growth and development of plants and causes mutations in DNA. Macroalgae such as Halopteris filicina contain multifunctional components that may improve plant tolerance to lead stress. In this study, seeds of tomato (Solanum lycopersicum) were subjected to six treatments comprising two levels of lead exposure (60 or 120 μM PbCl2) with or without H. filicina extract (0.5% in distilled water), a distilled water control, and a positive control (H. filicina extract) for 7 days. Physiological responses were investigated. Seedlings that had been treated with 60 and 120 μM PbCl2 without H. filicina extract showed root growth reduction of 55% and 68.6%, respectively, relative to the control, whereas for 60 and 120 μM PbCl2-treated seedlings with H. filicina extract applied, the reductions in root growth were lower, at 27.44% and 50.51%. The seedling viability index was decreased by 68.14% at 120 μM PbCl2 application without H. filicina extract, whereas a 42.48% reduction was recorded for 120 μM PbCl2-treated seedlings with H. filicina extract applied. Moreover, PbCl2 accumulation resulted in a decrease in leaf pigment content. Leaf pigment content was high in plants receiving the H. filicina extract. The rate of lipid peroxidation caused by PbCl2 was reduced with application of H. filicina extract. Genomic template stability was determined by using the inter simple sequence repeat-PCR technique, which revealed a decrease in DNA stabilisation with an increase in lead accumulation. However, this was alleviated by application of H. filicina extract. Our findings indicate that H. filicina extract both stimulates plant growth and protects from toxic effects by reducing accumulation of metals in the cell.