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Photomixed terahertz (THz) sources currently produce the most spectrally pure waves for frequencies spanning 300 GHz to 3 THz. This level of performance is crucial for enabling THz applications like high-speed wireless communications and molecular rotational spectroscopy. However, continuous-wave (CW) photomixed sources generate diminishing power above 300 GHz, which limits their practical utility: dropping from ~100 μW to ~10 nW at 1 THz. Resonant tunneling diodes (RTDs) offer a compact and energy-efficient alternative to photomixed THz sources, spanning the same frequency range. RTDs can generate milliwatts of THz power but are limited in application by a broad linewidth (poor spectral purity), limiting their use in applications requiring high coherence. A hybrid THz source combining the purity of photomixing and the power of RTDs can be realized via injection locking. Here we demonstrate injection locking of a waveguide-coupled RTD to a photomixed source. We characterize the residual phase noise of the injection locking process, the bandwidth limitations of injection locking, and show small-signal amplification of 40 dB. These measurements show a combined photomixed oscillator and waveguide injection-locked RTD amplifier at 260 GHz with state-of-the-art spectral purity. We anticipate these methods will be effective up to 1 THz.