The interaction of pyridine–Ag has been a hot topic since the finding of surface‐enhanced Raman scattering; however, there is still challenge to accurately evaluate the interactions within nano‐systems. Here we have synthesized chemically pure Ag18 nanoclusters by a facile method based on laser ablation in liquid and conducted a joint experimental and theoretical Raman spectroscopic study for pyridine adsorbed on the specific‐sized Ag clusters. As a result, altered Raman feature for the pyridine‐Ag18 system in contrast to pure pyridine is observed shedding light on the charge‐transfer interactions between pyridine and Ag. Utilizing natural population analysis, we studied the charge distribution before and after pyridine adsorbed on the Ag18 cluster. Also, we calculated static polarizability components and isotropic polarizability of pyridine and the pyridine‐Ag18 complex and studied the donor‐acceptor orbital overlaps via natural bond orbital analysis, charge decomposition analysis, electron density difference analysis, and density of state analysis. Such a comprehensive and semiquantitative study provides firm evidence for the chemical mechanism in surface‐enhanced Raman scattering theory. [ABSTRACT FROM AUTHOR]