"Maybe there are some categories of speech that have been historically unprotected, but have not yet been specifically identified or discussed as such in our case law," Roberts wrote, "but if so, there is no evidence that "depictions of animal cruelty" is among them."
Only Justice Samuel Alito dissented writing, "The Court strikes down in its entirety a valuable statue, that was enacted not to suppress speech, but to prevent horrific acts of animal cruelty"
The case has ignited debate about what kinds of speech should be protected.
The last time the Supreme Court addressed the issue was in 1982 when it carved out an exception to the First Amendment on the issue of child pornography.
In court briefs, the government links obscenity laws with animal cruelty and argues that neither merits protection from speech.
Solicitor General Elena Kagan wrote, "Like obscenity, the depictions are of patently offensive conduct that appeals only to the basest instincts."
But lawyers for Stevens argued that the case was not about dogfighting.
"What this case is about is whether Congress can create whole new categories of speech not protected by the First Amendment," said Stevens' lawyer Robert Corn-Revere. "It's creating a whole new category that has never been recognized as unprotected by the First Amendment."
Some First Amendment supporters have come to Stevens' defense, worrying about restrictions the government can place on speech.
In a brief filed by the Thomas Jefferson Center for the Protection of Free Expression, lawyers wrote that it is wrong for the government to equate protecting children from sexual abuse with the interest of protecting animals from inhumane treatment.
"Morals, values, religious beliefs, customs and laws compel adult Americans to provide far greater protection to children than they do to animals or even other adults," the group's lawyers wrote.
However, the Humane Society argued that Stevens' lawyers were trying to mislead people and that Congress dealt with the problem narrowly.
"The law at issue doesn't cover anything that has any journalistic, educational, artistic or social value," says the groups' lawyer, Jonathan Lovvorn. "When you strip away all the hysteria and rhetoric, this is a narrow law that only applies to those trafficking obscene materials over state lines."
Lovvorn said that even though Stevens claims he was against dogfighting, his efforts to depict images of it and sell them across county lines contributed to animal cruelty.
"If you dry up the interstate market for this material, it will reduce the underlying criminal activity," Lovvorn said.
ABC News' Devin Dwyer contributed to this report.