Rosie O'Donnell has belatedly apologized to Melania Trump for blasting out on social media her 10-year-old son, Barron, might be autistic ... which he is not.

Rosie now says, "I apologize to @MELANIATRUMP - i was insensitive in my RT - i am sorry for the pain i caused - it was not my intent - i am truly sorry."

Melania went ballistic after a YouTuber posted a video suggesting Baron had autism, and Rosie then re-posted the video with the caption, "Barron Trump Autistic?"

TMZ broke the story, Melania hired lawyer Charles Harder, who fired off a letter to the YouTuber telling him if he didn't take the video down he'd sue. He also called out Rosie in the letter, but did not threaten her.

Our sources say Melania has been indignant at Rosie for doing what she believes was a mean-spirited act.