"I won't participate in anything that's not first tier because we have a first tier campaign," the Kentucky senator told CNN's Wolf Blitzer.
Paul's announcement came moments before Fox Business Network, the sponsor of Thursday's debate, announced that just seven candidates had qualified for the main stage. Those candidates are Donald Trump, Ben Carson, Ted Cruz, Marco Rubio, Chris Christie, Jeb Bush and John Kasich.
Both Paul and Carly Fiorina, who appeared on the main stage in last month's CNN debate, were relegated to the undercard round, along with Rick Santorum and Mike Huckabee, according to the network.
Fox Business announced last month that its January 14 debate would include the top six candidates based on the average of the five most recent national polls, as well as those who finish in the top five in Iowa or New Hampshire. Paul failed to meet that criteria.
The Fox Business debate will mark the first time only seven candidates have appeared on the main stage for a Republican debate.
The undercard debate, meanwhile, will see no more than three candidates, given Paul's refusal to honor what he sees as "a mistake" by the debate's sponsor.
"It's a mistake because the thing is we actually have been in the top five or six in most of the recent polls. In fact, last week in a national poll we were just one point out of fourth place. So I think it's a mistake to try to exclude me from the national debate," he told Blitzer.
Following the interview, Paul's campaign released a statement in which it protested Paul's treatment by the media.
"By any reasonable criteria Sen. Paul has a top tier campaign," the statement read. "He will not let the media decide the tiers of this race and will instead take his message directly to the voters of New Hampshire and Iowa."
"Multiple national polls including CNN, CBS, Fox, Marist and others have him in 5th or 6th place, one had him just a single point out of 4th," the statement continued. "In multiple polls he scores ahead of Jeb Bush, Chris Christie, John Kasich and Carly Fiorina. In yesterday's Marist Iowa poll he was alone in 5th place. In fact his numbers are on the rise in recent weeks."
Fox Business did not make public which polls were under consideration prior to Monday night's announcement, and Fox Business spokespeople did not respond to requests for comment on Monday night.