Roger Stone, who reportedly remains "a confidant to Trump" following the campaign, said he had a "perfectly legal" way of communicating with Mr Assange, whose organisation released thousands of hacked emails from Hillary Clinton's campaign chairman John Podesta.
One of a series of profanity-filled tweets Mr Stone sent during an argument on Twitter said: "you stupid stupid b***h - never denied perfectly legal back channel to Assange who indeed had the goods on #CrookedHillary [sic]"
In another tweet to the same person, he said: "bring it! Would enjoy crush u in court and forcing you to eat s**t - you stupid ignorant ugly b***h ! [sic]"
Here are some things Trump advisor Roger Stone spent Saturday night saying to random people on Twitter pic.twitter.com/96k2QbIXwt— Liam Stack (@liamstack) March 5, 2017
Roger Stone has now deleted those two tweets where he called someone a stupid bitch, FYI.— Liam Stack (@liamstack) March 5, 2017
Mr Stone has called the FBI's investigation into whether there was improper contact between the Trump campaign and Russian officials a "witch hunt."
He suggested the FBI was monitoring his communications and told CBS News: “Sure they’ll get my grocery lists, they may get the emails between my wife and I, but here’s what they won’t get — any contact with the Russians."
He denied any connection to Russia and refuted allegations he knew WikiLeaks were going to publish emails stolen from Mr Podesta, adding: "It's a witch hunt. I know it is."
Trust me, it will soon the Podesta's time in the barrel. #CrookedHillary— Roger Stone (@RogerJStoneJr) August 21, 2016
Two months before WikiLeaks began publishing new emails from Mr Podesta everyday, Mr Stone tweeted: "Trust me, it will soon the Podesta's time in the barrel. #CrookedHillary"
US intelligence officials concluded "with high confidence" Russian President Vladimir Putin ordered the hacks of the Democratic National committee to help Mr Trump win the election.
It is not the first time Mr Stone has admitted to having "back channel communication" with Mr Assange.
Last year, he told CBS4 News: "I do have back-channel communication with Assange, because we have a good mutual friend.
"That friend travels back and forth from the United States to London and we talk. I had dinner with him last Monday."
However, he denied he had met or spoke to Mr Assange.
At the time, the Clinton campaign released a statement to say: “It is disturbing that Roger Stone, a longtime Donald Trump confidante, has confirmed the fact that he has “back-channel communications” with Wikileaks, an organisation that is working with the Russian government to affect the American presidential election.”