Tetramethylammonium hydroxide (TMAH) and substances that release tetramethylammonium (TMA) are classified as Priority Assessment Chemical Substances (PACSs) under registration number 17 of the Japan Chemical Substances Control Law (CSCL, 1973). This classification requires a thorough human health hazard assessment and derivation of Hazard Assessment Value (HAVs) for the oral and inhalation exposure at the Assessment II stage. We analyzed their general, developmental, reproductive toxicity, genotoxicity, and carcinogenicity using hazard data from both domestic and international risk assessment agencies and subsequently proposed an HAV. For oral exposure, a no-observed-adverse-effect-level (NOAEL) of 1 mg/kg/day, based on transient or persistent salivation in parent rats from a TMAH developmental and reproductive toxicology (DART) screening study, was chosen as the point of departure (POD). The POD was then divided by uncertainty factors (UFs) totaling 1,000 (interspecies variation: 10, intraspecies variation: 10, short study duration: 10), resulting in an oral HAV of 0.001 mg/kg/day for TMAH. Due to a lack of hazard data for humans and animals via inhalation, an HAV for the inhalation route was not established.