This research delves into the green synthesis of silver nanoparticles (Ag NPs) utilizing the aqueous extract of Foeniculum vulgare (fennel). The synthesis process involved mixing the fennel extract with an aqueous silver nitrate solution at two different temperatures, 25 °C and 60 °C, under ultrasonication. The synthesized Ag NPs were subjected to a comprehensive characterization using various analytical techniques. The UV-visible spectroscopy revealed a distinct absorption peak at 450 nm, indicative of the successful formation of Ag NPs. Fourier-transform infrared (FTIR) spectroscopy identified the presence of functional groups such as amides, amines, and flavonoids, which played a role in the reduction and stabilization of the nanoparticles. Powder X-ray diffraction (p-XRD) analysis confirmed the crystalline nature of the Ag NPs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses showed that the nanoparticles were predominantly spherical, with diameters ranging from 10 nm to 30 nm, irrespective of the synthesis temperature. The antimicrobial efficacy of the Ag NPs was evaluated, revealing inhibition zones of 26 mm and 17 mm against Escherichia coli at 25 °C and 60 °C, respectively. The Ag NPs demonstrated moderate antibacterial activity against Gram-negative bacteria and limited antifungal activity. These findings highlight Foeniculum vulgare as a viable and eco-friendly source for the synthesis of Ag NPs, with promising applications in antimicrobial treatments.