J.K. Rowling choked down some crow by apologizing for her rant about a viral video of President Trump seemingly ignoring a handshake from a disabled boy at the White House.

J.K. blasted the Prez for the video, saying ... "This monster of narcissism values only himself and his pale reflections." She added it was "horrible" he couldn't bring himself to shake the little boy's hand.

After the 'Harry Potter' author ranted, the boy's mother said the video was misleading -- and she wanted to make it clear Trump had not snubbed her son, and the boy wasn't even trying to shake the President's hand.

J.K. tweeted an apology to the mother and her son, saying she didn't intend to cause them any distress.

Interestingly, she did NOT apologize to Trump. You may have heard ... she's not a fan