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The interaction between hydrochlorothiazide (HCTZ), a diuretic drug, with human serum albumin (HSA) was investigated by different biophysical approaches such as UV absorption, circular dichroism (CD), Fourier transform infrared (FT-IR), and fluorescence spectroscopy in 50 mM sodium phosphate buffer, pH 7.4. The results of fluorescence titration experiments revealed that HCTZ strongly quenches the intrinsic fluorescence of HSA through a static quenching mechanism. Binding constants and the number of binding sites were calculated using Stern–Volmer plots. Thermodynamic analysis of the binding data elucidated that hydrogen bonding and van der Waals interactions played the major role in the binding process. Computation of the protein surface hydrophobicity (PSH) index using 1-anilinonaphtalene-8-sulfonate indicated that considerable decrement in PSH value of the protein happened upon drug binding. The binding site of HCTZ in HSA was identified using warfarin and ibuprofen as site markers, a result confirmed by molecular docking study. The results of CD experiments showed that some alterations occurred in the structure of the protein upon ligation. Also, the results of FT-IR experiments were in good agreement with CD experiments. It looks like that both secondary and tertiary structures of HSA have been affected upon HCTZ binding.Hydrochlorothiazide (HCTZ) binds to subdomain IIA (site I) on HSA structure.The surface hydrophobicity index of HSA was decreased upon HCTZ binding.Both secondary and tertiary structures of HSA have been affected upon HCTZ binding.