Many experts believe that the tech—which could deliver speeds that are 100 to 200-times faster than 4G—won't arrive at scale for at least another two years.
While a 5G smartphone has yet to be released, AT&T updated its phones to replace the "LTE" symbol commonly found atop of its consumer's mobile screens with "5Ge," which some say suggests that its phones are ready for the fifth generation of mobile tech.
The federal suit, filed in the U.S. District Court of New York, wants AT&T to remove its "5Ge" logo from both marketing and its phones.
AT&T and Sprint did not immediately respond to a request for comment.
Speaking with Ad Age last week, Sprint CMO Roger Solé described AT&T's marketing as "very deceiving."
"This is designed to make people think they have a 5G network and that is absolutely a lie," Solé said. "They're shooting themselves in the foot and diluting the 5G message. It's total confusion for the customers."
Solé went on to claim that AT&T marketing practices is devaluing 5G for the entire industry.
Both Verizon and T-Mobile have also called out AT&T for its marketing practices.
Last month at CES, AT&T Communications CEO John Donovan said AT&T phones aren't capable of 5G speeds, adding that the symbol was put on phones to "ready" consumers about the forthcoming switch to 5G.
"I have now occupied beachfront real estate in my competitor's head, and that makes me smile," Donovan said on stage at CES. "Every company is guilty in building a narrative in how they want the world to work and I love the fact we broke our industries narrative two days ago. They're frustrated."
Sprint was acquired by T-Mobile for $26 billion in April, though that deal has yet to receive regulatory approval. The two say the merger will help them create "the mother of all 5G networks," which will be critical for technologies such as augmented reality, autonomous cars, virtual reality and connected devices.