This letter proposes a novel wearable robot designed to assist with shoulder abduction using a double-petal mechanism based on petal fold origami driven by shape memory alloy (SMA)-based artificial muscle. The proposed double-petal mechanism consists of two petal structures that mimic the scapula and humerus, respectively. It follows the scapulohumeral rhythm to prevent bone collision and reduce the compressive force on the glenohumeral joint. The mechanism is designed to achieve high mechanical gain and torque output while minimizing the overall weight using lightweight SMA spring actuators and carbon fiber-reinforced plastic-based frames. The proposed robot can assist with shoulder abduction both with and without energy input using bundles of SMA spring actuators. It can generate assistance torque up to 6.36 Nm passively and 12.6 Nm actively at a 90° degree. To verify the assistance performance of the proposed robot, surface electromyography of the lateral deltoid is measured with and without the assistance of the robot and the results confirm that it effectively assists in shoulder abduction.