A possible ammonia leak in the cooling system had the crew from the American segment of the station move temporarily to the Russian segment.
It was a false alarm, NASA said.
Tests indicated that the off-scale measurements were the result of a computer failure, and not an ammonia leak.
International Space Station program manager Mike Suffredini said the relocation was done as a precaution because of four measurements that were off scale. These measures could have indicated an ammonia leak.
The astronauts, as trained, put on masks, moved to the Russian segment, and closed the hatch to the American segment, he said.
Two U.S. astronauts are aboard the International Space Station: commander Barry Wilmore and flight engineer Terry Virts.
Once it was determined that the emergency was a false alarm, the astronauts still had to wait while equipment on the U.S. segment was powered back up, Suffredini said.
NASA tweeted that at 3:05 p.m. ET, the hatch was reopened and the astronauts returned. There was no ammonia in the area, NASA said.
The crew was unable to carry out experiments that were planned, but no research was lost, Suffredini said.
The Americans were joined by Samantha Cristoforetti, a flight engineer with the European Space Agency, and others in the space station's six-member crew.
"Hey everybody, thanks for your concern," Cristoforetti tweeted. "We're all safe & doing well in the Russian segment."