A low-temperature low-pressure humidity chamber has been developed at the Korea Research Institute of Standards and Science for the calibration of radiosonde humidity sensors. The humidity generator operates in a two-temperature and two-pressure mode in which a saturator and a test chamber are separately immersed in two baths. The saturation performance of the saturator is first evaluated at a low temperature (−75 °C) and at atmospheric pressure. In this condition, the uncertainty of the frost-point temperature by the saturator is 53 mK with the coverage factor k  =  2. Then, a chilled mirror hygrometer is calibrated for the compensation of low-pressure effects such as a pressure drop along the tubing line inside the hygrometer. Significant differences between the frost-point temperature at the hygrometer inlet and that measured by the hygrometer are observed at a pressure lower than 20 kPa. An empirical formula for compensating the low-pressure effects on the hygrometer is obtained and, subsequently, is used for the validation of the frost-point temperature generated at a low temperature and low pressure in the test chamber. The corresponding uncertainty due to the calibration of the hygrometer including the compensation of low-pressure effects is 306 mK at k  =  2. A representative humidity generation and its validation by the hygrometer is demonstrated at a temperature (−70 °C) and two pressures (4.6 kPa and 20 kPa) in the test chamber. Uncertainty budgets on relative humidity in the test chamber are presented from 10 %rh to 40 %rh with a 10 %rh interval at both pressures. Consequently, the combined uncertainty is less than 2%rh (k  =  2) which includes uncertainty factors such as the saturation efficiency, the adsorption/desorption hysteresis, the validation by the hygrometer measurement at low pressures, and so on. The newly developed humidity chamber enables the calibration of radiosonde humidity sensors at low temperature and low pressure conditions that mimic upper air environments in a traceable manner to the International System of Units (SI).