The president insisted the story was ”totally fabricated” in response to Congresswoman Frederica Wilson’s claim he had made the “insensitive comments” before the funeral of Lance Sergeant David Johnson.
He was was one of four Green Berets killed by Islamist militants in Niger.
Mr Trump said the Florida Democrat who reported the phone call had intentionally lied about the incident.
He wrote on Twitter: “Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!”
Ms Wilson had told CNN she overheard the phone call between Mr Trump and Mrs Johnson and repeated her claim about the offensive comment.
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She said: “Basically he said, ‘Well I guess he knew what he signed up for, but I guess it still hurts’.
“I heard what he said because the phone was on speaker.”
She continued: “He said that. To me, that is something that you can say in a conversation, but you shouldn’t say that to a grieving widow.
“That’s so insensitive.”
When asked about Mr Trump’s tweet denying the claims Ms Wilson said the president was a “sick man”.
She said: “I don’t know what kind of proof he could be talking about – I’m not the only person that was in the car, and I have proof too.
“This man is a sick man, he’s cold-hearted and he feels no pity or sympathy for anyone.”