The law was developed before the Internet, though, and has not been updated to reflect the change in technology.
The new rule will change the definition a "retail pet store" to exclude outlets that sell pets over the phone or via internet, thereby making them subject to the Animal Welfare Act. Regulators say that the law's standards will ensure that animals are treated humanely before sale, whereas brick and mortar stores are already subject to the public's oversight.
Animal supporters say the measure will close a loophole that lets puppy mills go unregulated.
"They're usually puppy mills that are selling online and posing as small family breeders," said Kathleen Summers, the director of outreach and research for the Human Society of the United States' puppy mills campaign, "but they've been sort of allowed to categorize themselves as retail pet stores because they sell retail, meaning just directly to the public."
"Responsible breeders don't sell puppies to people they haven't met in person," she added, noting that the Humane Society estimates there are 10,000 puppy mills in the country which sell up to 4 million puppies a year.
"We think this will help thousands of dogs," said Summers.
The rule will also exempt more small breeders by increasing the number of breeding female dogs, cats and other species that an owner can have before being subject to the rules from three to four.
On Wednesday, the White House received the rule for a final analysis. It has 90 days to look it over and consult with stakeholders before issuing the regulation.