Ferdinand appeared to endorse a Tweet by another user of the social networking site which described Chelsea defender Ashley Cole, who gave evidence on behalf of Terry in the trial, as a "choc ice", a term understood to mean someone who is black on the outside but white on the inside.
Terry was found not guilty last month of racially abusing Ferdinand's brother Anton during a Barclays Premier League game between Chelsea and QPR on October 23 last year, although he is also now contesting a Football Association charge over the same incident.
The FA said in a statement today: "Manchester United's Rio Ferdinand has been fined £45,000 after an independent regulatory commission found proven a charge of improper conduct in relation to a comment posted on Twitter.
"The commission decided that the comment was improper and brought the game into disrepute. In addition, the commission found that the breach included a reference to ethnic origin, colour or race."
Cole, who also told the court he was a long-standing friend of the Ferdinand brothers, was accused by one person who sent a tweet to Rio Ferdinand of being a "choc ice".
The person wrote: "Looks like Ashley Cole's going to be their choc ice. Then again he's always been a sell out.
"Shame on him."
Manchester United defender Rio, who played with Terry at the heart of England's defence and whose parents attended every day of the five-day trial in support of his brother, replied: "I hear you fella!
"Choc ice is classic! hahahahahahha!!"
Users began discussing the comments and reacting to them and Ferdinand wrote: "And if I want to laugh at something someone tweets....I will! Hahahahaha! Now stop getting ya knickers in a twist!''
Terry was found not guilty of abusing Anton Ferdinand on July 13 at Westminster Magistrates' Court.
Watch: Ferdinand fined over 'choc ice' tweet.. More Videos
The Chelsea defender, who was stripped of the England captaincy while the case was ongoing, admitted using the phrase "f****** black c***" towards Ferdinand but said he was only sarcastically repeating what he thought Ferdinand had accused him of saying.
Terry was charged by the FA over the incident on July 27 and has requested a personal hearing, with no date set for the hearing to take place. The FA said he was free to continue representing England until the case had been heard.
Rio Ferdinand was not included in the England squad for Euro 2012, with national team manager Roy Hodgson citing "footballing reasons".
Sir Alex Ferguson believes the Football Association want to stop players courting controversy on Twitter.
Ferguson has stated on more than one occasion his complete bewilderment at the Twitter phenomenon.
However, at the unveiling of new signing Robin van Persie, who has in excess of 2.1million followers, Ferguson said the FA were out to make their mark.
"It was almost certain to happen," he said.
"They (the FA) want to close the door on this kind of thing.
"What surprises me is that other players have been twittering for ages and have never been challenged by the FA.
"His status in the game has caused that and who he plays for is maybe another issue."
Ferguson repeated his stance on Twitter, and confirmed United's present code of conduct strictly forbids any of his players from revealing details of club business.
"Twitter; I do not understand it," he said.
"I don't know why anybody can be bothered with it.
"But it is there and as a club we have given instructions to the players that they cannot talk about Manchester United.
"Rio is mature but he still has to remember that one word can make a difference - and you can't take it back."