The state's gay marriage ban was challenged by two gay couples -- one seeking to marry in Texas and one seeking to have their marriage, which was performed in Massachusetts, to be recognized.
The case appears to be headed for the U.S. Supreme Court. Texas Attorney General Greg Abbott (R), who is also seeking to become the state's next governor, is expected to appeal.
Texas Democrats hailed the decision.
"This is a historic day for the LGBT community and the state of Texas," state Democratic Party Chairman Gilberto Hinojosa said. "As Dr. King once stated, 'The arc of the moral universe is long, but it bends towards justice.' Today, all Texans can celebrate that we are one step closer to justice and equality for all."
Texas Gov. Rick Perry (R) said his state will indeed seek to uphold the ban.
"Texans spoke loud and clear by overwhelmingly voting to define marriage as a union between a man and a woman in our Constitution, and it is not the role of the federal government to overturn the will of our citizens," Perry said, adding: "We will continue to fight for the rights of Texans to self-determine the laws of our state."
Texas is the latest in a quick succession of states in which a judge has struck down a ban on gay marriage. Judges in Virginia, New Jersey, Oklahoma, Utah and California have also struck down those states' bans.
A judge in New Mexico also recently legalized gay marriage, though that state didn't have a ban in place. Judges in Ohio and Kentucky have instructed their states to recognize gay marriages performed elsewhere.
Seventeen states, not including Texas, currently allow gay marriage.
Texas voters remain split on gay marriage, with a recent poll showing 48 percent favor it and 49 percent oppose it.
Below is the full ruling:
Updated at 3:53 p.m.