On the basis of nonlinear downconversion, we propose how folded waveguide structures can be utilized as highly efficient, compact, and tunable sources of THz radiation. A folded waveguide arrangement allows us to exploit long-range efficient light-matter interaction at a compact geometrical footprint. Nonlinear THz generation in these systems is analytically modeled with results well supported by numerical simulations. For a practical device, based on the silicon photonics platform in combination with nonlinear organic materials, we show that the emitted THz radiation has a monochromatic continuous wave power on the order of tens of microwatts. The required optical input power is only 100 milliwatts. The THz emission frequency can be in situ tuned to any desired frequency in the 1-10 THz spectral range without spectral gaps. This unique property makes such integrated THz sources suitable for applications in THz spectroscopy and THz imaging.