Calling the claims as "blatantly inaccurate and completely ridiculous", the 41-year-old TV host also added that she is considering legal action against the people who spread the rumour, reported Ace Showbiz.
"Stories circulating online, claiming that I said my son Evan may not have autism after all, are blatantly inaccurate and completely ridiculous," she wrote on Twitter.
"Evan was diagnosed with autism by the Autism Evaluation Clinic at the UCLA Neuropsychiatric Hospital and was confirmed by the State of California (through their Regional Center).
"Continued misrepresentations, such as these, only serve to open wounds of the many families who are courageously dealing with this disorder. Please know that I am taking every legal measure necessary to set this straight."
McCarthy and her former husband John Asher welcomed their son in 2002. The boy was diagnosed with the disorder in 2005.