The union for Southwest Airlines flight attendants called the decision "dangerous" and "designed to make the lives of TSA staff easier, but not make flights safer."
The changes were made public by TSA Administrator John Pistole during an aviation conference in New York.
Starting April 25, passengers going through U.S. airports can bring on board Swiss Army-type knives -- specifically, ones with blades no longer than 2.36 inches.
This marks the first time such knives have been allowed on board since security was heavily increased in the aftermath of the Sept. 11, 2001, terror attacks.
Pistole told the audience that TSA screeners at the Los Angeles International Airport alone seized roughly 47 such knives a day over the last three months of 2012, according to Air Transportation World.
"Frankly, I don't want TSA agents to be delayed by these," he said.
The agency said the changes were made as part of its "overall risk-based security approach" and to align with the international standards and those of European countries.
Among the other items also allowed on board are lacrosse sticks, ski poles and small, souvenir baseball bats.
But Stacy K. Martin, president of Southwest Airlines' Flight Attendants Union, TWU Local 556, said in a statement that the decision should be "immediately rescinded."
"While we agree that a passenger wielding a small knife or swinging a golf club or hockey stick poses less of a threat to the pilot locked in the cockpit, these are real threats to passengers and flight attendants in the passenger cabin," she said.
The union represents 10,000 flight attendants for Southwest.
Under the TSA decision, however, box-cutter type knives used by the 9/11 hijackers are still prohibited. Razors as well as knives with molded grips also are still banned.