The allegations include that the Russian regime "has been cultivating, supporting and assisting Trump for at least five years".
Donald Trump has taken to Twitter to deny the claims, saying it's a "total political witch hunt".
He also linked to an article entitled "Fake News: BuzzFeed Runs 'Unverifiable' Trump-Russia Claims" which deemed the news site's decision to publish the documents a "shocking breakdown of journalistic ethics".
The dossier, understood by Buzzfeed to be written by a former British intelligence agent, also claims that Mr Trump was involved in "perverted sexual acts" during a visit to Moscow.
The document concludes that Trump's "unorthodox behaviour in Russia" had provided authorities with "enough embarrassing material ... to be able to blackmail him if they so wished".
WHAT ARE THE ALLEGATIONS?
Although the dossier is not verified, Buzzfeed said it chose to publish it "so that Americans can make up their own minds about allegations about the President-elect that have circulated at the highest levels of the US government".
It acknowledges the document includes some clear errors, including the misspelling of Alfa Group.
The unverified documents claim the Russian regime "has been cultivating, supporting and assisting Trump for at least five years".
It claims that Mr Trump and his inner circle "have accepted a regular flow of intelligence from the Kremlin, including on his Democratic and other political rivals".
According to the unverified document, sources have confided that the "Kremlin has been feeding Trump and his team valuable intelligence on his opponents, including Democratic presidential candidate Hillary Clinton, for several years".
The report also alleges that secret meetings took place in Moscow between Russian officials and a number of Trump surrogates, including his campaign manager Paul Manafort and his lawyer Michael Cohen.
The report alleges that Mr Cohen supposedly met with the Russians in Prague last August.
Mr Cohen has since denied these claims in a post that was retweeted by Mr Trump.
The allegations get even more outrageous. They suggest Mr Trump's conduct in Moscow included "perverted sexual acts" that were arranged and monitored by the Federal Security Service.
In the document, a source says Mr Trump hired the presidential suite of the Ritz Carlton Hotel in Moscow, where he knew President Obama and Michelle Obama had stayed on one of their officials trips.
The source goes on to say that Mr Trump asked prostitutes to perform lewd sex acts in their room.
"According to Source D... Trump's perverted conduct included... defiling the bed where they had slept by employing a number of prostitutes to perform a 'golden showers' (urination) show in front of him."
The dossier also suggests that Russian officials made various bribes in order to win influence over the celebrity billionaire, although there is no evidence that he accepted any offers.
It ultimately warns that "Trump's unorthodox behaviour in Russia over the years had provided the authorities there with enough embarrassing material on the now-Republican presidential candidate to be able to blackmail him if they so wished".
Again, these reports are unverified, and clear errors have been noted.
FBI TIGHT-LIPPED ON EXPLOSIVE CLAIMS
James Comey, the director of the FBI, has refused to say if the bureau is investigating this alleged link, The Guardian reports.
Testifying before the Senate intelligence committee today, he said: "I would never comment on investigations - whether we have one or not - in an open forum like this, so I really can't answer one way or another."
Sources told CNN these allegations prompted then senate Democratic leader Harry Reid to confront Comey in a letter in October, in which he wrote: "It has become clear that you possess explosive information about close ties and co-ordination between Donald Trump, his top advisers, and the Russian government - a foreign interest openly hostile to the United States."
According to CNN, the synopsis was partially produced from "memos compiled by a former British intelligence operative, whose past work US intelligence officials consider credible".
The classified briefings were presented by four of the most senior US intelligence chiefs - national intelligence director James Clapper, Mr Comey, CIA director John Brennan and NSA director-admiral Mike Rogers.
Multiple sources said intelligence chiefs included the synopsis in the briefing documents to ensure the President-elect knew the compromising information was circulating about him.
"I have a sense the outgoing administration and intelligence community is setting down the pieces so this must be investigated seriously and run down," one high-level official told CNN.
"I think (the) concern was to be sure that whatever information was out there is put into the system so it is evaluated as it should be and acted upon as necessary."
On Twitter, NBC reporter Tom Winter suggested the information had circulated to reporters as far as six months ago, but was never acted on.
It is unclear how many other media outlets may have been offered the information.
Several details of Mr Trump's financial information - including his tax returns and debts to foreign entities - remain unknown.
