The second-order nonlinear optical (NLO) properties of a push–pull monomer, as well as of its dimer composed of two monomeric units connected through a non-conjugated spiro junction, are investigated by means of hyper-Rayleigh scattering (HRS) and quantum chemical calculations. It is shown that dimerization to form a molecule with an orthogonal geometry enhances the first hyperpolarizability by circa 20 % and induces a decrease of the dipolar character of the HRS response. Theoretical calculations correlate well with experimental measurements and allow an in-depth rationalization of the impact of the spiro linkage on the NLO properties through a detailed analysis of the relevant electronic excited states.