Two astronauts transferred a 50-foot extension boom to the International Space Station, marking what may be the last mission by NASA in its role constructing the orbiting lab.
"Twelve years of building and 15 countries and now it's the Parthenon in the sky and hopefully the doorstep to our future," spacewalker Gregory Chamitoff before heading back inside. "So congratulations everybody on assembly complete."
Chamitoff said it was fitting for space shuttle Endeavour to be at the space station for the end of construction since it was there for the first assembly mission in December 1998.
Russia's space agency is expected to continue working on the space station and shuttling American astronauts to and from the lab.
The Endeavour crew is scheduled to return to Florida on Wednesday.
It is the last flight for the shuttle before it is retired to the California Science Center in Los Angeles.
The Associated Press contributed to this story.