This paper presents a Wake-Up Receiver for ultra-low-power IoT systems requiring robustness against temperature variations and false wake-ups. The former is accomplished by implementing a dedicated biasing block to ensure a roughly constant Analog Front-End input impedance and matching network gain over temperature. The latter is achieved thanks to a data-startable baseband logic featuring a Gated Oscillator Clock and Data Recovery circuit, which allows the reception of 256-bit codewords. A prototype was fabricated in an STMicroelectronics 90-nm CMOS technology; it receives 1-kbps OOK-modulated packets with a 433-MHz carrier frequency and consumes 54.8 nW with a 0.6-V supply voltage. The measured sensitivity at room temperature is -49.5 dBm with a 10-3 Missed Detection Ratio and its variation is 6 dB over a -40 °C to + 95 °C temperature range. Zero false wake-ups were detected transmitting random packets for 60 hours, resulting in an overall reduction in the IoT node energy consumption.