β-Cyclodextrin (β-CD) is studied as a carrier of the drug mianserin (MIA). β-CD with MIA adducts with 1 : 1 and 2 : 1 stoichiometry are investigated in vacuo and in water using quantum chemical methods: PM6 and B3LYP/6-31G(d,p). An effect of the dispersion correction GD2 and the basis set superposition error on the complexation energies is also evaluated. Additionally, the interaction between MIA hydrochloride and β-CD in aqueous solution at 298.15 K is examined experimentally by isothermal titration calorimetry. Interaction parameters, such as the binding constant, enthalpy, entropy and Gibbs free energy, are presented. Analysis of the obtained data led to the following conclusions: the interaction of MIA with β-CD is rather strong; there is no significant energetic difference between the 1 : 1 complexes of β-CD with S-MIA and R-MIA enantiomers; the 2 : 1 (β-CD : MIA) adduct is energetically more favorable than 1 : 1; the complex formation of MIA + β-CD is enthalpy and entropy driven.