<p>Millimetre-wave and terahertz signals enable high-performance applications; however, tunable integrated electronics remain limited beyond 100 GHz, restricting progress in wireless communication, high-resolution imaging, and spectroscopy. Photonic approaches can address this, albeit with increased size and power consumption. Here we describe a chip-scale, wide-band millimetre-wave frequency synthesizer, which uses integrated nonlinear photonics and high-speed photodetection to exploit the nearly limitless bandwidth of light. We generate dual, microresonator-soliton frequency combs whose interferogram is fundamentally composed of harmonic signals spanning the millimetre-wave and terahertz bands. By phase coherence of the dual comb, we precisely stabilize the interferogram to generate any output frequency from direct current to &gt;1 THz. Across this range, the synthesizer exhibits exceptional frequency stability, characterized by an Allan deviation of 3 × 10⁻¹² in 1-s measurements; and low phase noise, achieving −83 dBc Hz⁻¹ at 100 kHz offset and a reference frequency of 150 GHz. We use a modified uni-travelling-carrier photodiode with an operating frequency up to 500 GHz to convert the interferogram to an electrical signal, with continuously tunable tones. Our work harnesses the coherence, bandwidth and integration of photonics to extend the accessible frequency range of current, advanced-node complementary metal–oxide–semiconductor microwave electronics to the millimetre-wave and terahertz bands.</p>