"I don't know whether or not such collusion-and that's your term-such collusion existed, I don't know", he said.
Brennan also said he was anxious by "a number" of contacts between Russian Federation and people in the United States.
The investigations surrounding Russia's interference in the 2016 election, and whether any associates of President Donald Trump were involved, picked up steam over the last two weeks, after the president abruptly fired FBI Director James Comey.
Any possible collusion is now being pursued by the bureau, as well as separate House and Senate investigations.
Asked repeatedly whether he had seen evidence of collusion, he stressed that the CIA's business was intelligence rather than evidence and he could not make that judgment.
A separate but related incident demonstrated the possibility that Trump could have worked with the Russians due to negligence rather than intentionally.
He said that Americans don't always know when they're helping Russian spies because Russian intelligence agents often use intermediaries.
In his opening statement, Brennan described the brazenness of Russian interference in the 2016 election.
"This is something that's critically important to every American", he added. He said the head of the FSB denied that Russian Federation was meddling, but said he would raise the issue with President Vladimir Putin.
And he said there appears "very, very damaging leaks, and I find them appalling and they need to be tracked down". "Mr. Putin and Russian officials are trying to actively exploit what is going on now in Washington to their benefit and to our detriment".
Committee members also asked Brennan about Trump's revelation of highly classified information in his Oval Office meeting with two top Russian officials on May 10, and Brennan responded that Trump apparently violated two "protocols" required before classified information is revealed to foreign governments.
Rep. Devin Nunes, R-Calif., chairman of the committee, recused himself from the panel's Russian Federation investigations after the House Committee on Ethics announced it was investigating Nunes for disclosing classified information.
"One is that such intelligence, classified intelligence, is not shared with visiting foreign ministers or local ambassadors".
"Neither did it go in the proper channels nor did the originating agency have an opportunity to clear the language for it", Brennan said.
Testifying in Congress, former Central Intelligence Agency director John Brennan says he warned Moscow previous year against interfering in the United States presidential election. "I said if Russian Federation pursued this course it would destroy" improved relations between the two countries. Devin Nunes, a Republican from California, recused himself from the Russian Federation investigation after the House Committee on Ethics said it was investigating whether he disclosed classified information to the White House.
Brennan continued briefing the committee in closed session after the public hearing.
Comey's dismissal came in the middle of an Federal Bureau of Investigation probe into Russian meddling in the election and possible collusion with Trump's campaign, leading to accusations by Trump critics that the Republican president has sought to curtail the Federal Bureau of Investigation inquiry, which could potentially amount to obstruction of justice.