The 21-foot-long supply closet is packed with goods and a humanoid robot that will remained boxed up for another two months.
The space station and shuttle crews teamed up to attach the newest module, using a robot arm to anchor it.
The newest module was built in Italy and was named Leonardo after Leonardo da Vinci. It becomes the 13th room at the sprawling station and is the only one devoted entirely to storage.
Space station commander Scott Kelly thanked Mission Control for the new storage unit. "It's much needed," he said.
Discovery's mission has been extended a day and will land next Tuesday. It's the last flight for NASA's oldest shuttle.
Only two other shuttle flights remain. Endeavour will deliver a major science instrument for the station's exterior, and Atlantis will drop off one last load of spare parts and supplies to keep the outpost going until 2020.