The compartment will serve as a docking port for European and Japanese laboratories that will be delivered on the next three shuttle flights.
The grand opening came about a half hour ahead of schedule. Commander Peggy Whitson and astronaut Paolo Nespoli delayed their lunch so the event could happen before the station's orbit temporarily blocked the ability to send a video downlink to Mission Control.
After struggling a bit to unlatch the door, the pair slipped inside the new addition. Nespoli floated around the room while Whitson collected air samples. They wore the protective gear in case any debris was shaken loose inside the module during launch or installation.
"It's a pleasure to be here in this very beautiful piece of hardware," said Nespoli, who joined Discovery's crew to personally deliver the Italian-made pressurized chamber.
The other eight crew members aboard the orbiting complex set aside their work to watch the hatch opening, snapping photographs throughout the process.
Astronauts added the school bus-sized room - called Harmony - during a 6.5-hour spacewalk Friday, using a robotic arm to lift it from the shuttle's cargo bay and install it on the station.
A block of time had been set aside Saturday for more inspections of Discovery for launch damage. But NASA decided Friday that a focused examination wasn't necessary because the shuttle's thermal shielding appeared to be in good shape.
Examining the shield that protects the shuttle from the searing heat of re-entry has been a top priority for NASA since Columbia's loss in 2003.
Columbia disintegrated after a piece of foam broke off its external fuel tank during liftoff and gashed a wing, allowing hot gases to penetrate the ship during its descent. All seven of its astronauts were killed.
NASA examined images of Discovery gathered during launch, a second-day wing and nose scan and a pre-docking backflip maneuver before determining there was no significant damage. Another inspection two days before landing will check for any impacts from orbital debris.
Astronauts Daniel Tani and astronaut Scott Parazynski also were scheduled to spend time Saturday preparing for the next day's spacewalk, the second of a record-tying five outings planned for the mission.
They plan to install spacewalking handrails and other equipment to the outside of Harmony. The spacewalkers also will take a look at a rotary joint for the space station's solar wings that has exhibited electrical current spikes in the past 1½ months. The joint is necessary for turning the solar wings toward the sun.
More computer repairs are also on the crew's Saturday to-do list. Both Discovery and the space station have had some networking problems over the past few days.