A lightweight and low-cost unmanned aerial vehicle (UAV) system for coastal wave and tide measurements is developed. This system is based on an assembly of a multirotor UAV, a robotic lidar, an altitude and heading reference system (AHRS), and a real-time kinematic (RTK) Global Navigation Satellite System (GNSS). A great advantage of the system is that it can be operated at low altitude in a few meters; the accuracy and spatial resolution can therefore be increased. When the system was moved up and down in 2-12 m, the root-mean-square error (RMSE) was approximately 5 cm compared to the ground truth value measured by a manual RTK GNSS. The system was operated in a hover mode to measure the tide and waves in the field. The results of using the UAV-RTK lidar system were consistent with those of in situ measurements using a pressure sensor. The root-mean-square errors between the two techniques for measurements of tidal elevation, significant wave height, and wave period were 4.9 cm, 4.8 cm, and 0.028 s, respectively. This finding suggests the system could be applied to measure instantaneous sea surface displacement. The system provides the potential for using a low-cost, extremely portable, and efficient tool for monitoring wave properties, topographic changes, and water-level gradients in coastal zones.