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Herein, we report our success synthesizing silver nanoparticles (AgNPs) using aqueous extracts from the leaves and flowers of <i>Calotropis gigantea</i> growing in the geothermal manifestation Ie Seu-Um, Aceh Besar, Indonesia. <i>C. gigantea</i> aqueous extract can be used as a bio-reductant for Ag⁺→Ag⁰ conversion, obtained by 48h incubation of Ag⁺, and the extract mixture in a dark condition. UV–Vis characterization showed that the surface plasmon resonance (SPR) peaks of AgNPs-leaf <i>C. gigantea</i> (AgNPs-LCg) and AgNPs-flower <i>C. gigantea</i> (AgNPs-FCg) appeared in the wavelength range of 410–460 nm. Scanning electron microscopy energy-dispersive X-ray spectrometry (SEM-EDS) revealed the agglomeration and spherical shapes of AgNPs-LCg and AgNPs-FCg with diameters ranging from 87.85 to 256.7 nm. Zeta potentials were observed in the range of −41.8 to −25.1 mV. The Kirby-Bauer disc diffusion assay revealed AgNPs-FCg as the most potent antimicrobial agent with inhibition zones of 12.05 ± 0.58, 11.29 ± 0.45, and 9.02 ± 0.10 mm for <i>Escherichia coli</i>, <i>Staphylococcus aureus</i>, and <i>Candida albicans</i>, respectively. In conclusion, aqueous extract from the leaves or flowers of <i>Calotropis gigantea</i> may be used in the green synthesis of AgNPs with broad-spectrum antimicrobial activities.