James Woolsey, a former national security official under Republican and Democratic administrations who has been a fierce critic of President Barack Obama, said in a statement he will advise Trump "on the urgent need to reinvest in and modernize our military in order to confront the challenges of the 21st century."
"Mr. Trump's commitment to reversing the harmful defense budget cuts signed into law by the current administration, while acknowledging the need for debt reduction, is an essential step toward reinstating the United States' primacy in the conventional and digital battlespace," Woolsey said in a campaign statement. "Mr. Trump understands the magnitude of the threats we face and is holding his cards close to the vest."
Woolsey told CNN's "At This Hour" that he met with Trump on Friday and has agreed to advise the campaign "principally if not exclusively on national security matters such as defense, intelligence, maybe some aspects of energy ... whatever he needs."
One reason Woolsey said he supports Trump is that the Republican presidential nominee "seems willing to keep a secret and not to blab everything to the public and our opponents."
His hire comes a week after Trump vowed to end the sequester on defense spending and boost military spending, including increasing troop levels and boosting the number of US military ships and planes.
Woolsey, who chairs the Foundation for Defense of Democracies, a neoconservative group, most recently advised Texas Sen. Ted Cruz, Trump's chief rival for the Republican nomination.
Woolsey also served as a top US arms control negotiator in the 1980s.
The former CIA director in his campaign statement slammed Democratic nominee Hillary Clinton's leadership at the State Department under the Obama administration as he explained his decision to join Trump's campaign.
Woolsey argued Clinton "demonstrated a complete lack of understanding and an inability to lead the agency she headed in such a way as to maintain its mission and security."
"Based on the emails thus far released we know that Secretary Clinton also lacks the ability to lead her senior managers while complying with and maintaining the basic protocols designed to protect our government's sensitive and classified information," Woolsey said.
There is no evidence that Clinton's personal email account, which she used for government business, was hacked.