This paper presents the analysis and design of an ultra-low-power injection-locked transmitter supporting ON-OFF keying (OOK) and frequency-shift keying (FSK) modulation for short-range sensor node applications. To achieve sub-μW operation, this design incorporates an enhanced start-up crystal oscillator, allowing for aggressive duty cycling of the entire transmitter. The duty-cycling signal is generated by a nanowatt on-chip clock oscillator, effectively minimizing sleep-mode power consumption. Frequency tuning of injection-locked oscillators enables operation across multiple standard sub-GHz bands, including the MedRadio band at 400 MHz and the ISM bands at 433 MHz and 915 MHz—within a single architecture, which, to the best of the authors’ knowledge, has not been previously reported. With an optimized injection phase sequence, the carrier generation block, operating with a stabilized reference signal, achieves a sub-11 ns settling time. To prove the concept, the transmitter is fabricated in 130 nm technology, occupying a core area of 0.133 mm². Operating from a 0.25—0.9 V supply, the transmitter delivers an output power of −23.3 dBm at 432 MHz. At 1 Mb/s, it consumes 183 μW in continuous operation and only 435 nW in duty-cycled mode with a duty cycle of 0.09%. With optimized start-up and switching, the transmitter achieves a data rate of 85 Mb/s and an energy efficiency of 2.57 pJ/bit— representing, to the best of the authors’ knowledge, the highest energy efficiency and data rate among sub-GHz ultra-low-power transmitters reported to date.