Background/Objectives: “Hermes” is an ankle–foot orthosis (AFO) with negative stiffness designed to mechanically compensate the symptomatic increase in plantarflexion (PF) torque (i.e., ankle joint torque resistance to dorsiflexion, DF) in patients with spastic paresis. Methods: The effectiveness of “Hermes” was evaluated in twelve patients with chronic unilateral spastic paresis after stroke. Using a robotic ankle manipulator, stiffness at the ankle joint was assessed across three conditions: ankle without Hermes (𝐴A), ankle with Hermes applying no torque compensation (𝐴+𝐻0%A+H0%), and ankle with Hermes tuned to compensate 100% of the patients’ ankle joint stiffness (𝐴+𝐻100%A+H100%). Results: A significant reduction in PF torque was found with Hermes applying compensation (𝐴+𝐻100%A+H100%) compared to the conditions without Hermes (𝐴A) and with Hermes applying no compensation (𝐴+𝐻0%A+H0%). Furthermore, a significant reduction in positive dorsiflexion work was found with Hermes applying compensation (𝐴+𝐻100%A+H100%) compared to the condition with Hermes applying no compensation (𝐴+𝐻0%A+H0%). Hermes did not significantly contribute to additional PF torque or positive work when applying no compensation (𝐴+𝐻0%A+H0%). Conclusions: The reductions in PF torque achieved with Hermes are comparable to those seen with repeated ankle stretching programs and ankle robot training. Thus, Hermes is expected to assist voluntary dorsiflexion and improve walking in patients with spastic paresis.