A lower court denied Twitter's motion in June and ordered the company to handover the information.
The judge, noting that Harris's tweets were public, wrote, "There can be no reasonable expectation of privacy in a tweet sent around the world."
He also argued that all third-parties have the burden of deciding if subpoenas are legitimate and that Twitter shouldn't receive a special exception.
The American Civil Liberties Union (ACLU) filed a brief supporting Twitter's appeal.
“Under the First and Fourth Amendments, we have the right to speak freely on the Internet, safe in the knowledge that the government can’t get information about our speech without a warrant and without satisfying First Amendment scrutiny," Aden Fine, an ACLU attorney, wrote in a blog post. "We’re hopeful that Twitter’s appeal will overturn the criminal court’s dangerous decision, and reaffirm that we retain our constitutional rights to speech and privacy online, as well as offline.”
A spokeswoman for the Manhattan District Attorney's Office declined to comment.