A cooperative regenerative braking control algorithm is proposed for a six-speed automatic-transmission-based parallel hybrid electric vehicle (HEV) during a downshift that satisfies the requirements for braking force and driving comfort. First, a downshift strategy during braking is suggested by considering the re-acceleration performance. To maintain driving comfort, a cooperative regenerative braking control algorithm is developed that considers the response characteristics of the electrohydraulic brake. Using the electrohydraulic brake’s hardware and an HEV simulator, a hardware-in-the-loop simulation (HILS) is performed. From the HILS results, it is found that the proposed cooperative regenerative braking control algorithm satisfies the demanded braking force and driving comfort during the downshift with regenerative braking.