Comey said that conclusion is "a high-confidence judgement of the entire intelligence community and the members of this committee have seen the intelligence."
"It's not a close call," he said. "That happened. That's about as unfake as you can possibly get. It is very, very serious, which is why it's so refreshing to see a bipartisan focus on that."
In the widely anticipated hearing, Comey recounted a series of conversations that he says made him deeply uneasy and concerned about the blurring of boundaries between the White House and a law enforcement agency that prides itself on independence.
In his testimony, Comey's first public statements since his May 9 dismissal, the former FBI director detailed demands for loyalty from the president, and requests to end an investigation into embattled adviser Michael Flynn, who resigned as head of the National Security Agency.
Comey told the panel the issue of Russian interference goes beyond party politics.
"We have this big messy wonderful country where we fight with each other all the time, but nobody tells us what to think, what to fight about, what to vote for except other Americans," he said.
"We're talking about a foreign government that using technical intrusion, lots of other methods tried to shape the way we think, we vote, we act. that is a big deal, and people need to recognize it. It's not about Republicans or Democrats. They're coming after America, which I hope we all love equally.
"They want to undermine our credibility in the face the world. They think that this great experiment of ours is a threat to them. So they're going to try to run it down and dirty it up as much as possible. That's what this is about, and they will be back."