Astronauts Joe Tanner and Heidemarie Stefanyshyn-Piper installed solar panels on the orbiting ISS during nearly seven hours in space.
"This is another challenging and successful day," said John McCullough,
NASA's leading flight director. "This is day seven of a very tough mission," he said.
The "extra-vehicular activity" - NASA-speak for spacewalk -completed the series of three spacewalks for the 11-day mission.
Mr Tanner and Ms Stefanyshyn-Piper, the only woman in the Atlantis crew of six, deployed a radiator on the ISS to prevent overheating of the new solar array.
The two performed the mission's first spacewalk on Tuesday, using a robotic arm to attach a new 16-tonne structure bearing the solar arrays that Atlantis delivered to the ISS.
On Wednesday, two Atlantis astronauts spent seven hours in space loosening the solar arrays from restraints that prevented damage when Atlantis lifted off from Cape Canaveral, Florida.
On Thursday, astronauts stretched out the new solar array measuring 73 meters on the ISS. It will provide one-fourth of the power ISS will need when the orbiting laboratory is completed in 2010.
The new solar system will be activated on the next scheduled shuttle mission, by Discovery, in December.