Some studies have suggested that high ambient temperatures are a risk factor for kidney-related diseases. However, none have examined the association between ambient temperature and acute kidney injury (AKI). This study aimed to examine the association between daily mean temperature and AKI and identify high-risk subgroups in this association. We obtained health insurance claim data from the Health Insurance Review and Assessment Service (HIRA) for 24,800 admitted cases of AKI during the period 2007-2014 in Seoul, Korea. Using a time-series design and piecewise linear regression models, we estimated the percentage change in AKI admissions associated with daily mean temperature after controlling for relevant covariates. Daily mean temperature and AKI admissions displayed an inverse association in the cold season and a nonlinear relationship with a flexion point around 28.8°C in the warm season. AKI admissions increased by 23.3% (95% confidence interval [CI]: 14.3, 33.0) per 1°C increase in mean temperature above the 28.8°C flexion point in the warm season. The estimates were greatest among men with hypertension (55.1%; 95% CI: 25.1, 92.2). However, we did not observe significant increases in AKI admissions associated with ambient temperature in the cold season (0.4% [95% CI: -0.1, 0.9] per 1°C decrease in mean temperature). Our results suggest that hospital admissions for AKI increase in association with high temperature, particularly among men with hypertension in the warm season. Thus, early detection of AKI during heat wave periods is crucial. Our findings also provide evidence for the local government to target populations vulnerable to high ambient temperatures.