The US President said it was not true he made the comments to Myeshia Johnson in a phone call to express his condolences over the death of her husband, Sgt La David Johnson.
Mr Trump reportedly said: “Well I guess he knew what he signed up for but I guess it still hurt.”
His remarks were recounted by Democrat Representative Frederica Wilson who said that Mr Trump was must have a “brain disorder”.
The President tweeted that Miss Wilson “fabricated” the quotes to score political points.
The ugly dispute raised tensions once again between Mr Trump and the military in the US.
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Despite claiming to be a strong supporter of soldiers and veterans, the President has become embroiled in a series of rows that have tested the faith of front line troops in their commander-in-chief.
Mr Johnson, 25, a father-of-three from Miami Gardens, Florida, was one of four Special Forces soldiers killed by Islamic State militants during an ambush in Niger on October 4.
Mr Trump took nearly two weeks to call the families to express his condolences.
Miss Wilson said that she was in the car on the way to Miami airport to meet her husband’s coffin when Mrs Johnson received the five minute call from the President.
She added: “Basically he said: ‘Well I guess he knew what he signed up for but I guess it still hurt’.
Miss Wilson added: “You know, that is something that you can say in a conversation but you shouldn’t say that to a grieving widow. Everyone knows when you go to war you could possibly not come back alive, but you don’t remind a grieving widow of that. That is so insensitive. So insensitive.”
Miss Wilson also claimed that the President did not even remember Mr Wilson’s name and left his widow in tears.
She said: “This gentleman has a brain disorder, and he needs to be checked out. We should be praising his family, not insulting them.”
Mr Trump fired back on Twitter: “Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!”
Mr Johnson’s mother Cowanda Jones-Johnson said that she heard the same comments from the President.
A White House spokesman said: “The President’s conversations with the families of American heroes who have made the ultimate sacrifice are private.”