Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a memory stack, a semiconductor layer, a supporting structure, a spacer structure, and a contact structure. The memory stack includes interleaved conductive layers and dielectric layers and includes a staircase region in a plan view. The semiconductor layer is in contact with the memory stack. The supporting structure overlaps the staircase region of the memory stack and is coplanar with the semiconductor layer. The supporting structure includes a material other than a material of the semiconductor layer. The spacer structure is outside the memory stack and is coplanar with the supporting structure and the semiconductor layer. The contact structure extends vertically and is surrounded by the spacer structure.