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The green synthesized Ag/rGO composite nanoparticles were successfully synthesized using the Hummers' method with Amaranthus viridis extract. The localized surface plasmon resonance (LSPR) behavior of the Ag/rGO was studied using the Kretschmann configuration consisting of prism/Au/Ag/rGO/air. The addition of the Ag/rGO composite to the prism system aims to determine the effect of rGO on the SPR angle. X-ray diffraction results showed that there were four diffraction peaks with the presence of lattice planes (111), (200), (220), and (311), proving the face-centered cubic crystal structure of silver. The characterization results showed that the size of Ag NPs was 23.1 nm, and that of Ag/rGO composite was 16.7 nm. The EDX results of the Ag/rGO composite showed the presence of several elements such as C, O, Ag, and N. Fourier transform infrared analysis showed the presence of O–H, C–H, CO, CC, and C–O–C functional groups confirming the formation of Ag/rGO. The UV–Vis spectra showed an absorption peak at 257 nm and 346–412 nm with an increase in band gap energy from 3.27 eV to 3.40 eV. Meanwhile, the LSPR measurements demonstrate a shift in the SPR angle due to the addition of rGO, in which the angle shifts from 0.33 ° to 0.96 °. In conclusion, the addition of rGO can optimize the plasmonic properties of Ag, so that it potentially could be applied to enhance the performance of SPR-based biosensor applications.