We present a solid-state tunable terahertz source that exploits the theory of coupled oscillators to simultaneously achieve high output power and frequency tuning. Our proposed structure effectively generates and combines high power harmonics from multiple synchronized solid-state oscillators in a loop configuration. We study the dynamics of the system, find the stable modes, and show how the structure can dynamically select a desired coupling mode. Using this method, we fabricated 0.29 and 0.32 THz tunable sources with peak output powers of 0.76 and 0.5 mW both in a standard 65 nm bulk complementary metal-oxide semiconductor technology. This power level is around 10000 times larger than the state of the art which demonstrates that the proposed concept achieves significantly higher output power for a given solid-state process.