p-tert-butyl phenol (ptBP) is an alkylphenol organic compound that is used in the production of polycarbonates, phenolic resins, epoxy resins, etc., as a polymer chain terminator. However, it can occasionally be an impurity contaminant during the chemical synthesis of active pharmaceutical ingredients. Some alkylphenols are known as carcinogens or endocrine modulators, therefore controlling the impurity level to below the permitted daily exposure (PDE) value based on the toxicological risk assessment of ptBP should be considered. Here, we propose the PDE using toxicological information primarily obtained from OECD Existing Chemicals Database. Four scenarios for the oral PDE calculation were investigated, and the lowest value, 2 mg/day, was proposed as the most appropriate control criterion of ptBP based on the depigmentation toxicity seen in black mice. The PDE for the parenteral route was concluded to be 1 mg/day by applying a default bioavailability of 50% per ICH Q3D guideline.