Sudden unexpected deaths in bathtubs among elderly Japanese adults occur predominantly during the cold season. This study investigated the relationship between these deaths and bathing day temperature among elderly adults in Tokyo.Data for 1408 cases of bath-related deaths from January 1 to December 31, 2015 were obtained from the Tokyo Medical Examiner's Office. We excluded 409 cases for the following reasons: criminal death, injury-related death, suicide, intoxication, non-sudden death, not bathtub-related death, out-of-bathroom death, subject aged under 65 years, undetermined bathing date, institutional housing, and bathing not at subject's home. Ultimately, 999 cases were analyzed. Daily mean temperature data were collected. A time-series regression study was performed to estimate the influence of sex, age, and bathing day temperature. Monthly changes in the population bathing in a bathtub were considered in the model.The relative risk (RR) of sudden unexpected death in a bathtub was 1.381 for males (95% confidence interval [CI]: 1.218-1.564) compared to females. The RRs were 4.182 (95% CI: 3.523-4.986) and 9.382 (95% CI: 7.836-11.273) among those aged 75-84 years and ≥85 years, respectively, compared to among those aged 65-74 years. The RR increased to 1.092 (95% CI: 1.082-1.102) as the daily mean temperature decreased by 1 °C.Sudden unexpected death in a bathtub correlated with bathing day temperature among elderly Japanese adults, and extremely low temperature, male sex, and older age increased the risk of such death. Our findings provide insight into preventing sudden unexpected deaths in bathtubs.