In this work, we demonstrate a metamaterial (MM)-based SiN x –Si interlayer coupler. A gradient index (GRIN) MM was employed to achieve refractive index matching, balancing the large difference in the refractive index between Si (∼3.42) and SiN x (∼2). In addition, a two-layer SiN x interlayer coupler was proposed to further minimize the coupling loss and interlayer crosstalk. Finally, we demonstrated a three-dimensional (3D) SiN x –Si interlayer coupler with the −0.6 dB insertion loss per layer propagation, with the 40 nm 1-dB-bandwidth (1530–1570 nm) and <−45 dB interlayer crosstalk. These promising results demonstrate the great potential of a GRIN MM-based SiN x –Si interlayer coupler in 3D photonic integration.