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The technology of physicochemical surface modification is available for enhancing the bioactivity and osseointegration capability of tetragonal zirconia polycrystal (TZP). Hydrophobicity index and electrical charge play important roles in protein adsorption. We previously studied the mechanism underlying the adsorption of bovine serum albumin (BSA) on the surfaces of dental materials and hydroxyapatite in vitro. The aim of the present study was to clarify the correlation among the adsorption of BSA to TZP and physicochemically modified TZP surfaces and the zeta potential of BSA and TZP. We used TZP that was sintered at 1350°C for 2 h in air because this kind of TZP is widely applied in the field of dentistry. Surface physicochemistry was modified with ultraviolet light (UV) and atmospheric-pressure plasma treatment. The zeta potentials were measured with ELSZ-1000 and ELSZ-2000 analyzers (Otsuka Electronics, Hirakata, Japan). All experiments were conducted in 10 mM NaCl (pH 7.0). The zeta potentials of as-received TZP and BSA were negative, but those of UV- and plasma-treated TZP were positive. The reason the zeta potentials of TZP changed positive by physicochemical modification is due to an increase in the amount of basic hydroxyl groups. The zeta potentials of UV- and plasma-treated TZP after BSA adsorption were negative. These results suggested that electrostatic interactions play an important role in BSA adsorption to TZP and modified TZP surfaces, so that this modified surface may control the adsorption of protein.