Mexico has an extensive diversity of plants used for medicinal purposes. Green synthesis of gold nanoparticles (AuNPs) with medicinal plants has attracted significant attention because of advantages like ease, cost-effectiveness, eco-friendly, and rapid synthesis. This article presents the antioxidant activity and the ability to synthesize AuNPs of the aqueous and ethanolic extracts of Tagetes erecta L flowers (marigold) and Hibiscus sabdariffa L (Rosell) flowers. The synthesized AuNPs were characterized using UV–Vis spectroscopy, Scanning transmission electron microscopy (STEM), TGA analysis, FTIR, XDR, and a stability study. UV–Vis spectrum shows the surface plasmon resonance peak (SPR) around 500–580 nm. STEM images showed a spherical, triangular, and hexagonal morphology with diameters between 11.48 and 43.93 nm for aqueous extracts and 9.22 and 11.59 for ethanolic extracts of T. erecta and H. sabdariffa, respectively. TGA and FTIR analysis revealed that organic compounds present in extracts capped the surface of AuNPs. Characteristics and stability of green AuNPs were compared with chemical synthesis, observing that green AuNPs present lower diameters and better stability related to the capping compounds.