Prosecutors had asked a Salzburg court to place the girls, aged 16 and 17, in investigative custody pending an investigation into whether they were members of a terrorist organisation.
The teens had returned to Austria from Romania, where they were picked up by authorities on a train on December 30 as they tried to make their way to Syria to marry Islamic State jihadist fighters there.
But the Salzburg judge decided to release the girls.
"In her opinion the girls' behaviour is not criminal, not yet, because they were stopped in Romania and did not really get to join a terrorist organisation," a court spokeswoman said.
Prosecutors, who had wanted the suspects jailed to prevent them from fleeing, have 14 days to appeal against the ruling while the investigation continues.
Around 170 people, many from Islamic immigrant backgrounds, have travelled to the Middle East from Austria to join Islamist militant groups, the government says.
Around 60 have returned, raising fears they could launch attacks akin to the slayings in Paris last week.
The girls were detained in their homes in Salzburg and Upper Austria province after their return. They had Bosnian and Chechen family backgrounds, according to the APA news agency.
© Thomson Reuters 2015