Giffords wore a padded medical helmet and walked unassisted up the stairs of the plane.
She has been recovering at a Houston hospital since January after being shot in the head at a political event in Tucson, Ariz. on Jan. 8.
Giffords husband, astronaut Mark Kelly, is the commander of the space shuttle Endeavour crew. Endeavour will blast off Friday afternoon for its final mission.
Endeavour and its six-man crew are bound for the International Space Station. They will deliver a $2 billion physics experiment and a load of spare station parts. Four spacewalks are planned during the 14- to 16-day mission.
The congresswoman will watch the liftoff in private and will return to the Houston hospital.
President Barack Obama will arrive on launch day with his wife and two daughters in what will be the first visit by a first family for a launch. Only two other sitting presidents have ever witnessed a manned launch: Richard Nixon for Apollo 12 in 1969 and Bill Clinton for John Glenn's return to orbit in 1998 aboard shuttle Discovery.
The Associated Press contributed to this report.