Trump said it applied to the next 90 planes. The F-35 currently has a per-unit cost of about $100 million, which the company pledged to bring down even before Trump became involved.
"I appreciate Lockheed Martin for being so responsive," Trump told reporters at the White House.
Since his electoral win, Trump has criticized Lockheed and Boeing for the cost of government contracts. Earlier this month, Lockheed CEO Marillyn Hewson said the company was "very close to a deal" to lower the cost of the fighter program.
In a statement Monday, Lockheed Martin said it shares Trump's commitment to "delivering this critical capability for our men and women in uniform at the lowest possible cost to taxpayers." The company did not address how Trump got to the $600 million figure.
On Friday, Defense Secretary James Mattis ordered a review of the F-35 program to "determine opportunities to significantly reduce the cost."
The total cost of the program is an estimated $400 billion.
Lockheed shares traded slightly higher on Monday.