He also refused to take a question from a CNN reporter.
Trump called BuzzFeed a "failing pile of garbage," arguing the online media outlet "will suffer the consequences." A day earlier, BuzzFeed published a 35-page dossier of unsubstantiated allegations of Russian information-gathering on Trump.
At the news conference, Trump took CNN to task for "going out of their way to build it up," then refused to take a question from CNN's Jim Acosta.
"Your organization is terrible," the president-elect told Acosta at the nationally televised news conference at Trump Tower. "I am not going to give you a question, you're fake news."
A spokesperson for CNN released a statement, arguing that the network's reporting on the matter differed from BuzzFeed:
"CNN's decision to publish carefully sourced reporting about the operations of our government is vastly different than Buzzfeed's decision to publish unsubstantiated memos. The Trump team knows this. They are using Buzzfeed's decision to deflect from CNN's reporting, which has been matched by the other major news organizations. ... We made it clear that we were not publishing any of the details of the 35-page document because we have not corroborated the report's allegations."
Incoming White House press secretary Sean Spicer opened Trump's news conference with a take down of BuzzFeed.
"It's frankly outrageous and highly irresponsible for a left-wing blog, that was openly hostile to the president-elect's campaign, to drop highly salacious and flat-out false information on the internet just days before he takes the Oval Office," Spicer said.
Trump takes the oath of office at his inauguration on Jan. 20.
The Russian government on Wednesday called the BuzzFeed report published Tuesday night a "total hoax" and "pulp fiction."
BuzzFeed Editor-in-Chief Ben Smith on Tuesday evening tweeted an email he had sent to this staff.
It read in part: "As we noted in our story, there is serious reason to doubt the allegations. We have been chasing specific claims in this document for weeks, and will continue to."
Spicer on Wednesday had his own characterization of the decision to publish the report. "It's sad and pathetic attempt to get clicks. The report is not an intelligence report — plain and simple."
"For all the talk lately about fake news, this political witch hunt by some in the media is based on some of the most flimsy reporting, and is frankly shameful and disgraceful," he said.
Shortly after Spicer's remarks, BuzzFeed emailed CNBC, saying: "We stand by our decision to publish the dossier," referring readers to Smith's tweet.
Disclosure: CNBC parent NBCUniversal is a minority investor in BuzzFeed.