Bioreduction of metal ions for the synthesis of stable nanoparticles (NPs) in physiological environment has been a great challenge in the field of nanotechnology and its application. In the present study, well-defined biofunctionalized gold nanoparticles (AuNPs) were developed following a biomimetic approach for an enhanced anticancer activity. The fucoxanthins-capped crystalline AuNPs showed a particle size of 14 nm. The temperature-mediated biosynthesized NPs were characterized by UV–vis, dynamic light scattering, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The cytotoxicity of the NPs was analyzed on liver (HepG2) and lung (A549) cancerous cells. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay infers that the biofunctionalized polyshaped AuNPs synthesized with an aqueous macroalgae extract showed a satisfactory anticancer effect on the cell lines, as evaluated by changes in cell morphology, cell viability, and metabolic activity. An altered cellular function and the morphology of cancer cell lines suggest a potential for in vivo application of AuNPs and the need to understand the interactions between nanomaterials, biomolecules, and cellular components. With continued improvements, these NPs may prove to be potential drug delivery vehicles for cancer therapy.