The energies, geometries and harmonic vibrational frequencies of 1:1 5-hydroxytryptamine-water (5-HT-H2O) complexes are studied at the MP2/6-311++G(d,p) level. Natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM) analyses and the localized molecular orbital energy decomposition analysis (LMO-EDA) were performed to explore the nature of the hydrogen-bonding interactions in these complexes. Various types of hydrogen bonds (H-bonds) are formed in these 5-HT-H2O complexes. The intermolecular C4H55-HT···Ow H-bond in HTW3 is strengthened due to the cooperativity, whereas no such cooperativity is found in the other 5-HT-H2O complexes. H-bond in which nitrogen atom of amino in 5-HT acted as proton donors was stronger than other H-bonds. Our researches show that the hydrogen bonding interaction plays a vital role on the relative stabilities of 5-HT-H2O complexes.