Density functional theory has been utilized to compute the electronic and optical characteristics of zBNNRs (w = 6 and 8) doped with lithium. The results suggest that the simulated nanoribbons display properties similar to those of semiconductors. Furthermore, the graphs demonstrate that a 4% lithium doping level decreases the bandgap. The presence of lithium alters the dielectric function of boron nitride nanoribbons (BNNRs) by acting as a donor atom, thereby introducing additional electronic states within the energy bandgap. Additionally, the dopant enhances the static refractive index, particularly in the z-direction. In the energy range of 0–6 eV, both pristine zBNNRs (w = 6) and zBNNRs (w = 8) satisfy the criteria for the transverse-electric mode. Conversely, beyond 3.87 [for Li-doped zBNNRs (w = 6)] and 3.81 eV [for Li-doped zBNNRs (w = 8)], the lithium-doped nanoribbons support the transverse-magnetic plasmons.