We theoretically investigate the open-shell singlet nature and σ-/π-conjugation effects on the longitudinal second hyperpolarizabilities (γ) of one-dimensional chains involving silicon–silicon bonds, that is, polysilane and poly­(disilene-1,2-diyl), by comparison with their carbon analogues, polyethylene and polyacetylene, respectively. It is found that poly­(disilene-1,2-diyl) has less bond length alternation than polyacetylene and that σ-conjugation of polysilane is less effective on the enhancement of γ than π-conjugation of polyacetylene, whereas π-conjugation of poly­(disilene-1,2-diyl) indicates a more than 20 times greater enhancement of the γ than that of polyacetylene, which is known to be a typical nonlinear optical molecule with large γ, for one-dimensional chains involving 3–5 double bonds. Further theoretical analyses of poly­(disilene-1,2-diyl) reveal the σ- and π-electrons contribute negatively and positively to the γ, respectively. The latter contribution is significantly larger than the former and thus causes the remarkable enhancement of γ amplitudes due to the emerging open-shell singlet nature in the long π-conjugation length.