Most female soldiers report that in-service body armour systems are too large. We investigated whether a smaller prototype body armour system could improve thoracolumbar range of motion (ROM) and reduce interference when female soldiers performed dynamic postures. 97 female soldiers completed three ROM tasks and seven dynamic postures wearing no armour, an in-service body armour system, and a smaller prototype system. Feedback on comfort of the prototype system was also obtained. Thoracolumbar ROM and dynamic posture completion were both hindered by using body armour, although the participants’ performances were significantly less impeded when they wore the smaller prototype system compared to the in-service system. A smaller body armour system that is better matched to the anthropometric dimensions of female soldiers appears to improve overall fit and function. An increased range of body armour sizes and female-specific designs should be systematically explored to further enhance fit and function of body armour.