Therapeutic antibodies for subcutaneous (SC) injection must be formulated at high concentrations because of the large therapeutic dose and volume restriction. However, concentrated antibody solutions have undesirably high viscosity, which hampers SC injection. In this study, we demonstrated that hydantoin and its derivatives suppressed the viscosity of concentrated antibody and bovine serum albumin solutions. Hydantoin derivatives, in particular 1-methylhydantoin, appeared more effective. Both hydantoin and 1-methylhydantoin suppressed the viscosity of proteins more effectively when combined with a physiological concentration of NaCl. Moreover, hydantoin rings exhibited thermodynamically favorable interactions with hydrophobic amino acids, as demonstrated using solubility measurements. Molecular dynamics simulations indicated planar stacking interaction or T-shaped interaction between the hydantoin ring structure and the aromatic rings of tryptophan. Thus, the effects of hydantoin compounds in the presence of NaCl on the high viscosity of concentrated protein solutions result from the combined effects between hydantoin and NaCl in suppressing multiple interactions (electrostatic, hydrophobic, π–π, and cation−π interactions) between protein molecules. The obtained data here should be useful for developing therapeutic antibody formulations.