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: "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!"