The N-Dubz rapper made the remark while lashing out against one of his followers, telling them that he would make their "face f**ked".
He wrote: "Ur a d**khead in your HOOD you've never BEEN KNOWN...I'll get your Face F***ed DOWN SYNDROME."
The message, which was deleted shortly afterwards, has attracted criticism from the Down's Heart Group, which provides information about heart conditions that can affect sufferers.
"People with Down's syndrome have such a lot to offer," the charity's information officer Sarah Smith told The Sun.
"There will be a lot of Dappy's fans who have Down's syndrome who will not be happy he used that term as a form of abuse."
The 'No Regrets' star yesterday (July 19) appeared in court on affray charges stemming from an alleged petrol station fight in February.
He has also recently caused controversy with his 'Tarzan Part 2' video, which included a reference to the man convicted of killing headteacher Philip Lawrence.
The clip sparked a feud between Dappy and his cousin Tulisa Contostavlos, who accused him of having "lost his mind" and branded him a "little s**t".