A collapsible humidifier reservoir assembly is configured to humidify a flow of pressurized respiratory gas. The humidifier reservoir assembly includes a bottom portion comprising a heat conducting base plate and a top portion that is releasably secured to the bottom portion. A seal is sandwiched between the top and bottom portions. An air inlet is configured to receive the flow of pressurized respiratory gas, and an air outlet is configured to discharge the flow of pressurized respiratory gas from the humidifier reservoir assembly. The bottom and top portions are formed from a flexible material, and the humidifier reservoir is configured to expand upon being filled with fluid.