The Friday ruling involves a lawsuit led by Texas and 19 other states.
Federal judge Reed O'Connor sided with the states' argument that all of the Affordable Care Act is unconstitutional due to a recent change in federal tax law.
California and several other states argued that overturning Obamacare would mean millions of people lose their health insurance.
The ruling could be appealed to the U.S. Supreme Court.
You can read the judge's full ruling here.