Urea can serve as nitrogen source for coral holobionts and plays a cruscial role in coral calcification, although the degradation of urea by coral symbionts is not fully understood. In this study, we investigated the urea utilized pathway and the responses of the Symbiodiniaceae family to urea under high temperature conditions. Genome screening revealed that all Symbiodiniaceae species contain the urease (URE) and DUR2 subunit of urea amidolyase (UAD) system. However, only three speciesCladocopium goreaui, Cladopium c92, and Symbiodinium pilosum possess a complete UAD system, including both DUR1 and DUR2. Phylogentic analyses revealed that the UAD system in Symbiodiniaceae clusters more closely with symbiotic bacteria, indicating that horizontal gene transfer of UAD system has occured in coral symbionts. Physiology analysis showed that the symbiodiniacean species Cladocopium goreaui, which containing both URE and UAD, grew better under urea than ammonium conditions, as indicated by higher maximum specific growth rates. Furthermore, most genes of Symbiodiniaceae involved in urea utilization appeared to be stable under various conditions such as heat stress (HS), low light density, and nitrogen deficiency, wheras in ammonium and nitrate transporters were significantly regulated. These relatively stable molecular regulatory properties support sustained urea absorption by Symbiodiniaceae, as evidenced by an increase in δ15N2-urea absorption and the decreases in δ5N-NO3-, and δ15N-NH4+ from cultural environment to Symbiodiniaceae under HS conditions. Token together, this study reveals two distinct urea utilization systems in coral ecosystem and highlights the importance of the urea cycle in coral symbionts when facing fluctuating nitrogen environment in future warming ocean.