U.S. District Judge Dolly Gee said the U.S. Department of Justice relied on a "tortured interpretation" of an agreement that limits detention of immigrant children to 20 days.
Monday's ruling dealt a significant blow to the Trump administration's "zero tolerance" immigration policy.
Last month President Trump signed an executive order that attempted to end the policy of family separation and replace it with indefinite detention for immigrant families who crossed the border illegally.
Thousands of children -- including around 50 under 5-years-old -- still have not been reunited with their parents because of a policy the president of the American Academy of Pediatrics called "government-sanctioned child abuse."