The CIA chief defended his agency and said in a Sunday interview with Fox News' Chris Wallace that the CIA was not trying to harm the incoming President or his team.
"There is no interest in undermining the President-elect and the team coming in. It's our responsibility to make sure they understand exactly the dangers that are out there," Brennan said.
Trump has slammed the intelligence community repeatedly over the past few weeks in response to what he said was unfair coverage caused by leaked information from intelligence officials. On Sunday evening he even pointed fingers directly at the CIA director over Twitter.
".@FoxNews 'Outgoing CIA Chief, John Brennan, blasts Pres-Elect Trump on Russia threat. Does not fully understand.' Oh really, couldn't do...much worse - just look at Syria (red line), Crimea, Ukraine and the build-up of Russian nukes. Not good! Was this the leaker of Fake News?" he wrote.
In response to the President-elect's tweets the CIA told CNN late Sunday evening it had no comment.
First on Twitter and then at his first news conference since July, Trump likened the leaks to "Nazi Germany." Brennan called the Nazi reference "outrageous."
"I do take great umbrage at that, and there is no basis for Mr. Trump to point fingers at the intelligence community for leaking information that was already available publicly," Brennan said.
Brennan bristled at Trump's public displays of contempt for the intelligence community and said Trump needed to understand his words could undermine national security.
"If he doesn't have confidence in the intelligence community, what signal does that send to our partners and allies, as well as our adversaries?" Brennan said.
The CIA director and former deputy national security adviser to Obama also warned Trump on Russia, saying he did not believe Trump understood the threat of Russia, while also saying he hoped the Trump administration would be able to see an improvement in the US-Russia relationship.
"I don't think he has a full appreciation of Russian capabilities, Russia's intentions," Brennan said, adding that Trump "does not yet" fully appreciate what embracing Russia might mean.
And with just a few days remaining in office, Brennan looked ahead to the incoming national security team, naming Trump's picks for secretary of defense, CIA director and secretary of homeland security.
"They are the ones that are going to be able to give him some wise counsel about what he needs to do and not be very spontaneous," he said. "It's more than just about Mr. Trump. It's about the United States of America."