ABSTRACT: A novel method (the ‘gyro method’) was developed to monitor the attitude and dynamic acceleration of free-moving aquatic animals with high precision and temporal resolution (100 Hz) using a new type of animal-attached data logger with a 3-axis gyroscope, a 3-axis accelerometer, and a 3-axis magnetometer. The gyro method was applied to green sea turtles Chelonia mydas in both tank and open-sea experiments, and the data for 2 types of activity (steady swimming and breathing) were compared to those obtained by a conventional method with only an accelerometer and a magnetometer, using low-pass filtering or running mean smoothing to determine gravity-based acceleration. Significant differences (mean ± SD: 0.034 ± 0.012 g and 1.964 ± 1.090° for steady swimming) were found in the reconstructed gravity-based acceleration and attitude between the gyro method and the conventional method. The gyro method reconstructed the fluctuation in attitude associated with the power stroke of the sea turtles, which was not documented by the conventional method. Additionally, the dynamic acceleration determined by the conventional method differed from the true dynamic acceleration, whereas the gyro method was able to reconstruct any attitude change (and therefore dynamic acceleration) without using the vague criteria involved with low-pass filtering or running mean smoothing in the conventional method. Hence, our new method will be useful for accurately measuring kinematics and movement performance involving complicated changes in motion, such as foraging for prey and escaping from predators, or the interactions of endangered species with fishing gear.