Shares of the aerospace company dropped more than 4 percent in early trade, but then recovered some of those losses, ending the day about 2 percent lower. After the tweet, the company's market value initially dropped $4 billion. The impact of Trump's tweet, per character, was more than $28 million.
Trump tweeted that "billions of dollars can and will be saved on military (and other) purchases" once he takes office in January.
Lockheed Martin spokesman Bill Phelps said the company has invested large sums of money to reduce the cost of the F-35 stealth fighter program by 60 percent, correcting an earlier company estimate of 70 percent.
The company added it will welcome "the opportunity to address any questions the president-elect has about the program."
When Trump's transition team was asked by reporters about Trump's tweet, the team said a clear message is being sent that Trump will fight for taxpayers.
After the tweet, shares of defense stocks Boeing and General Dynamics dipped 0.72 percent and 2.87 percent, respectively. Both stocks later pared those losses as Boeing ended the day 0.43 percent higher, while General Dynamics shares were 0.94 percent lower on the session. Defense ETF's and iShares U.S. Aerospace & Defense declined more than 1 percent.
A week before Trump won the election, the Pentagon and Lockheed concluded negotiations on their ninth contract for F-35 fighter jets after 14 months of negotiation on the deal of more than $6.1 billion, Reuters reported.
Last week, Trump took a shot at Boeing via Twitter threatening to cancel its 747 Air Force One program. He claimed the total cost of a new 747 would be more than $4 billion. The company took a $1.4 billion hit to its market value.
CNBC's Jim Cramer said on "Squawk on the Street" on Monday that Trump's behavior is not typical of what previous presidents have done.
"I mean the president ... does he have the ability to say, 'Listen, we're going to take fewer product?' He does," He said. "Does he have the ability to say, 'Listen, we don't like these prices?' Historically, that's not what the president has done. People within the Pentagon have negotiated, but very high level."
Howard Rubel, Jefferies' managing director, told CNBC on Monday the cost of building a weapon defense program involves much more oversight than in the commercial world.
"I am sure there will be a list of how come things cost so much, and then you look at the engineering involved or the quality required to protect a solider. It will be an understanding of the balance versus the cost," he said.
Even with Monday's declines, Lockheed Martin's stock has gained more than 16 percent this year.
—Reuters and CNBC's Steven Kopack contributed to this report.
Correction: This story has been revised to reflect Lockheed Martin's correction that its cost savings are estimated at 60 percent, not 70 percent.