This study aimed to synthesize gold nanoparticles (AuNPs) using Terminalia catappa (TC) and Hippocratea excelsa (HE) extracts as reducing agents and a triblock copolymer Pluronic (F127 and P103) as the stabilizing agent. The formation of AuNPs was monitored by UV-visible spectroscopy, showing a signature peak between 530 and 540 nm, which corresponds to the Surface Plasmon Resonance (SPR) of AuNPs. As time progressed, the AuNPs synthesized only with the plant extracts showed a decrease in absorbance attributed to particle agglomeration. However, the addition of the copolymer in the reaction changed the behavior of the SPR, and the absorbance level remained almost constant, suggesting stability. This behavior was corroborated by Transmission electron microscopy (TEM). Micrographs showed well-separated spherical nanoparticles when TC extract was used (10-50 nm). However, AuNPs synthesized with HE extract showed spherical templates on whose surface smaller NPs were formed (~10 nm). The hydrodynamic diameter obtained by Dynamic light scattering (DLS) suggests that the biomolecules of the extract and polymer are attached to the nanoparticles. In addition, polyphenols and sesquiterpenes, responsible molecules for reducing Au3+ and copolymer signals were identified by Fourier transform infrared (FTIR). The results suggest that the copolymer promotes the stability of colloidal solutions and could modify the morphology of AuNPs.