In this study, we propose a method of prediction of sit-to-stand movement for an assist system. The proposed method enables the prediction of sit-to-stand movement before the buttocks leave the seat. To verify the accuracy of the proposed method, we conducted evaluation experiments on five subjects to determine whether the movement is sit-to-stand or another movement. The evaluation results showed that the movement was predicted with an average accuracy exceeding 95%. Furthermore, we developed a sit-to-stand movement assist system using the proposed method.