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."