We report the demonstration of growing two-dimensional (2D) hexagonal-AlN (h-AlN) on transition metal dichalcogenide (TMD) monolayers (MoS2, WS2, and WSe2) via van der Waals epitaxy by atomic layer deposition (ALD). Having atomically thin thickness and high theoretical carrier mobility, TMDs are attractive semiconductors for future dense and high-performance 3D IC, and 2D hexagonal boron nitride (h-BN) as a gate dielectric is known to significantly improve TMD device performance. However, h-BN growth requires 1000 °C temperature that is not compatible with CMOS fabrication, and ALD deposition of any high-k 2D insulator on TMD continues to be an elusive goal. The epitaxial 2D layered h-AlN by low-temperature ALD is characterized by synchrotron-based grazing-incidence wide-angle x-ray scattering and high-resolution transmission electron microscopy. In addition, we demonstrate the feasibility of using layered h-AlN as an interfacial layer between WS2 and ALD HfO2. The significantly better uniformity and smoothness of HfO2 than that directly deposited on TMD are desirable characteristics for TMD transistor applications.