Pictures of the 31-year-old looking somewhat frail and dishevelled with a can of Red Bull in his hand have begun circulating online, sparking concern amongst fans.
However, the Home Alone star's representative has insisted that he is "perfectly" healthy and denounced any news providers suggesting otherwise.
"Macaulay Culkin is in perfectly good health," they told E! "For Entertainment Tonight - or any other media outlet - to speculate otherwise is reckless, irresponsible and destructive. We implore them to discontinue the rumour."
It has been suggested that Culkin's weight loss is in preparation for a role in British film Service Man, though his publicist denied the actor's involvement.
Culkin separated from his girlfriend Mila Kunis in January 2011 after eight years together.
> Mila Kunis: 'I like sarcasm and self-deprecation in men'