John Brennan says an investigation into possible ties between Trump campaign officials and Russia is "well-founded".
He confirmed that his agency, foreign intelligence service the CIA had commissioned a special group mid-last year to investigate Russian intervention.
Speaking in Washington, Mr Brennan told the House intelligence committee he was the first US official to call out the Russians for their activities.
"Now the Russians are watching very carefully what's going on in Washington and they will try to exploit it for their own purposes and to see whether or not they can further seed partisan animosity here in Washington and try to roil the waters - the political waters - here. And so even though the election is over I think Mr Putin and Russian intelligence services are trying to actively exploit what is going on now in Washington to their benefit and to our detriment."
The former CIA director says Russia was engaged in a "very aggressive" and "wide-ranging effort" to interfere.
Mr Brennan added that, in August, he warned the head of the Russian intelligence agency to stop the meddling.
He says he was told that message would be passed on to President Vladimir Putin.
"It should be clear to everyone that Russia brazenly interfered in our 2016 presidential election process and that they undertook these activities, despite our strong protests and explicit warning that they not do so."
Mr Brennan believes there were enough contacts between Trump's campaign and Moscow to justify further investigation by the US domestic intelligence agency, the FBI.
But when asked repeatedly whether he had seen evidence of collusion, he repeated the role of the CIA is to gather intelligence, not evidence.
"I don't know whether or not such collusion -- and that's your term, such collusion -- existed. I don't know. But I know that there was a sufficient basis of information and intelligence that required further investigation by the Bureau to determine whether or not US persons were actively conspiring, colluding with Russian officials."
Meanwhile, the Senate intelligence committee says it will continue to seek testimony from dumped national security advisor Michael Flynn.
A key witness, General Flynn declined to comply with a subpoena for documents to the committee's Russia probe by invoking his Fifth Amendment right against self-incrimination.
Committee vice-chairman Democratic Senator Mark Warner says two subpoenas will now be issued to Michael Flynn's businesses.
"While we disagree with General Flynn's lawyers' interpretation of taking the Fifth (amendment) it is even more clear that a business does not have a right to take a Fifth if it's a corporation, so those subpoenas - one has been served, one is in the process of being served - and we keep all options on the table but we think the committee's moved forward aggressively."
Senate committee chairman Republican Richard Burr says General Flynn could face contempt charges if he does not comply.
"Well, we've taken actions that we feel are appropriate right now. If in fact there's not a response, we will seek additional counsel advice on how to proceed forward; at the end of that option is a contempt charge. And I've said that everything is on the table. That's not our preference today. We would like to hear from General Flynn. We'd like to see his documents. We'd like him to tell his story, because he publicly said 'I've got a story to tell.' We're allowing him that opportunity to do it."
