Flexible temperature sensors can be attached to the surface of human skin or curved surfaces directly for continuous and stable data measurements, and have attracted extensive attention in myriad areas. Carbon nanomaterials possess great potential for temperature sensing, and flexible temperature sensors based on carbon nanomaterials have demonstrated unique advantages such as high sensitivity, fast response, good mechanical adaptability, low-cost fabrication processes, high cycling stability and reliability. In this review, the working mechanisms, device structures, material compositions, fabrication technologies, temperature sensing properties, the crucial roles of carbon nanomaterials, specific advantages and existing limitations of different types of flexible temperature sensors based on carbon nanomaterials are comprehensively elaborated and discussed. Based on recent advances, conclusions are made and challenges as well as future perspectives are systematically outlined and discussed.