In this paper, an ultra-low power, high accuracy CMOS smart temperature sensor customized for clinical temperature monitoring based on substrate p-n-p bipolar junction transistors (BJTs) is presented. A power efficient analog front end with a sensing-range customized multi-ratio pregain stage is proposed to effectively utilize the input range of the incremental analog-to-digital converter to relax the conversion speed and resolution requirement. A block-based data weighted averaging technique is also proposed to achieve highly accurate pre-gain ratios while significantly reducing the implementation complexity. The complete temperature sensor is implemented in a standard 0.18 μm CMOS process occupying an active area of 0.198 mm. Measurement results from 20 test chips show that an inaccuracy of ±0.2 °C (3σ) is achieved from 25 °C to 45 °C after one-point calibration. The average power consumption is 1.1 μW at a conversion speed of 2 Sa/s.