The space lab, known as Kibo, the Japanese for "hope", will be the largest module attached to the station so far.
Among Discovery's crew of seven is Japanese astronaut Akihiko Hoshide, who will oversee the installation of the 11.2-metre-long Kibo.
Once operational Kibo will accommodate up to four scientists and act as Japan's first permanent foothold in space.
A third and final component of the lab is due to fly on a later shuttle mission.
Last and largest of four research modules for International Space Station
Japan's first permanent manned presence in space
To be delivered into orbit by three shuttle flights
Will conduct experiments on effect of microgravity
Includes platform with robotic arm to expose experiments to space
Also carried on Discovery is a replacement pump which astronauts hope will fix the space station's Russian-built toilet.
The toilet broke down several weeks ago, and astronauts have had to make do with a cumbersome manual system to dispose of waste.
Until the repair is complete, the three-member station crew will use the shuttle's toilet - or they will use extra emergency bags that Discovery is also bringing.
Discovery took off from Florida on Saturday using a new design of external fuel tank designed to prevent a repeat of the deadly debris strike that caused the Columbia disaster in 2003.
During the launch several pieces of foam were seen falling off the giant tank, but Nasa officials say they do not believe any of them were a cause for major concern.
Nonetheless, the shuttle will undergo further inspections of its crucial heat shields before it returns to Earth.