Thermal transport in multilayers (MLs) has attracted significant interests and shown promising applications. Unlike their single-component counterparts, MLs exhibit thermal conductivity that can be effectively engineered by both the number density of the layers and the interfacial thermal resistance between layers, with the latter being highly tunable via the contrast of acoustic properties of each layers. In this work, we experimentally demonstrated ultralow thermal conductivity of 0.33 ± 0.04 Wm-1K-1 at room temperature in MLs made of Au and Si with a high interfacial density of 0.2 interfaces nm-1. The measured thermal conductivity is significantly lower than the amorphous limit of either Si or Au, and is also much lower than previously measured MLs with similar interfacial density. With a ratio of the Debye temperatures of Au and Si of ~3.9, the Au/Si MLs represent the highest mismatched system in inorganic MLs measured to date. In addition, we explore the prior theoretical prediction that full phonon dispersion could better model the interfacial thermal resistance involving materials with low Debye temperatures. Our results demonstrate that MLs with highly dissimilar Debye temperatures represent a rational approach to achieve ultralow thermal conductivity in inorganic materials and can also serve as a platform for investigating interfacial thermal transport.