It has suspended all flights operated by the fleet of 51 jets while the turbine blade crack is investigated.
The fault was discovered during a routine check of one of the planes at Edwards Air Force Base in California.
It is the second time in two months the fleet has been grounded.
Pentagon spokeswoman Kyra Hawn says it is not known how long the planes will be out of service for this time.
"We're talking about analysis of one blade in one turbine," she said.
"We have to find out through structural tests whether or not this is an isolated incident or whether they will have a more far reaching impact on the F-35 fleet."
The F-35 is the Pentagon's biggest weapons program, known as the Joint Strike Fighter (JSF) program, but it has been dogged by delays and cost increased.
Australia has delayed its order of the fighter jets after originally indicating it would buy 100.
Former defence minister Joel Fitzgibbon has sharply criticised top Australian military officials over what he calls their "obsession" with the F-35 fighters.
Mr Fitzgibbon, now the Federal Government's chief whip, says a fixation on the purchase has led to an ad hoc approach to Australia's air capability.
"I think there is an almost obsession with the JSF within the uniformed ranks - this is their brand new toy," he said.
"They were determined to secure four squadrons of the JSF and they were fond of running interference on anything that might threaten their capacity to secure those four squadrons."