Objectives: The aim of this study was to develop an estimation equation for energy expenditure during water walking based on the acceleration and walking speed. Design: Cross-validation study. Methods: Fifty participants, males (n = 29, age: 27–73) and females (n = 21, age: 33–70) volunteered for this study. Based on their physical condition water walking was conducted at three self-selected walking speeds from a range of: 20, 25, 30, 35 and 40 m/min. Energy expenditure during each trial was calculated. During water walking, an accelerometer was attached to the occipital region and recorded three-dimensional accelerations at 100 Hz. A stopwatch was used for timing the participant’s walking speed. The estimation model for energy expenditure included three components; (i) resting metabolic rate, (ii) internal energy expenditure for moving participants’ body, and (iii) external energy expenditure due to water drag force. Results: When comparing the measured and estimated energy expenditure with the acceleration data being the third component of the estimation model, high correlation coefficients were found in both male (r = 0.73) and female (r = 0.77) groups. When walking speeds were applied to the third component of the model, higher correlation coefficients were found (r = 0.82 in male and r = 0.88 in female). Good agreements of the developed estimation model were found in both methods, regardless of gender. Conclusion: This study developed a valid estimation model for energy expenditure during water walking by using head acceleration and walking speed.