Real-time instantaneous vital sign monitoring systems help medical staff in the intensive/high care unit to monitor a patient’s condition, but the data are usually discharged and not stored. This vital sign data may exhibit trends that could be useful information for the medical staff to estimate a patient’s prognosis. However, to make an accurate estimation, a large amount of data should be acquired and analyzed; therefore, long-term trend monitoring systems and analyses are required to provide better medical treatment. This study aims to develop a centralized long-term monitoring system for heart rate variability of a patient undergoing treatment in the intensive/high care unit. The long-term monitoring system for heart rate variability was examined in the laboratory. It was capable of acquiring and storing vital sign data from two clients running simultaneously. The server system received the data and performed a real-time analysis before presenting the results as a long-term trend, thus enabling the medical staff to observe and monitor the heart rate variability trends in patients. In conclusion, a long-term monitoring system for heart rate variability in intensive/high care unit patient has been developed that can retrieve, store, and analyze vital sign data from more than one patient to track changes in their condition.