Although hexagonal ice (ice Ih) is the most common and highly studied crystalline form of ice, its nucleation from clathrate hydrates is poorly understood. Here, we report the formation of ice Ih through the dissociation of formaldehyde hydrate, prepared under ultrahigh vacuum (UHV) in the temperature window of 130-135 K. This unique route for crystallization is highly facile, and it occurs below the usual crystallization temperature of ice Ih of 155 K in UHV; the associated activation energy is also lower. Time-dependent reflection absorption infrared spectroscopy was used to detect the formaldehyde hydrate and to determine the crystallization kinetics of ice Ih. The dissociation of formaldehyde hydrate is found to be a diffusion-controlled process, which reduces the activation barrier of crystallization. This study provides evidence that formaldehyde hydrate can exist in extremely low (P, T) conditions without forming the geminal diol or its polymer in the presence of water. This new and facile route of crystallization in the context of interstellar environments may have implications for cometary and prebiotic science. © 2021 American Chemical Society.