We have designed and realized an athermal 4-channel wavelength (de-)multiplexer in silicon nitride (SiN). Minimized thermal sensitivity is achieved in a wide wavelength range by using wide and narrow waveguides with low and different thermal-optic coefficients in the two arms of Mach–Zehnder interferometers (MZIs). The SiN core layer and ${\mathrm{SiO}}_2$ cladding layers are deposited by a low-temperature plasma-enhanced chemical vapor deposition process. The fabricated MZI filter exhibits a thermal sensitivity within $\pm 2.0\mathrm{pm}/°\mathrm{C}$ in a wavelength range of 55 nm to near 1300 nm. Then, an athermal (de-)multiplexer based on cascaded MZIs has been demonstrated with a crosstalk $\le 22\mathrm{dB}$ and a thermal sensitivity $<4.8\mathrm{pm}/°\mathrm{C}$ for all four channels, reduced by 77% compared to a conventional SiN (de-)multiplexer. Owing to the passive operation and compatibility with the CMOS backend process, our devices have potential applications in 3D integration of photonics and electronics.