"This was, in my opinion, made up," Trump told CNN after Thursday night's Republican debate. "Everybody said nothing happened. Perhaps she made the story up. I think that's what happened."
Fields has alleged that Trump campaign manager Corey Lewandowski grabbed her and bruised her while she was trying to ask Trump a question after a press conference on Tuesday night.
Ben Terris, a reporter at The Washington Post, said he witnessed the incident and Lewandowski "grabbed her arm and yanked her out of the way,"
bruising her and causing her to stumble.
Those claims caused outrage Thursday among reporters, dozens of whom criticized the Lewandowski for grabbing Fields and the Trump campaign for violent tactics.
But on Tuesday night, Trump said he didn't believe the incident had happened.
"I heard that nothing happened. You know, we're surrounded by Secret Service. We have many Secret Service. You see some of them here. When we left, I spoke to them. Nothing happened. This was, in my opinion, made up," he told CNN.
"Now I didn't see anything. All of a sudden we heard about it later on. But the Secret Service said nothing happened. The Secret Service are amazing people. They said absolutely nothing happened. He [Corey Lewandowski] didn't hear about it until like, the next day," Trump continued.
"I wasn't involved in it. But the Secret Service was surrounding everybody. They said nothing happened. Everybody said nothing happened. Perhaps she made the story up. I think that's what happened," he said.
When CNN asked Lewandowski about the incident, he declined to answer. "I'm going to work with Mr. Trump tonight," he said.
When asked if he had touched Fields at all, Lewandowski said: "Did you hear what I said?"
He later tweeted that Fields is "delusional" and that he had never met her before.
Fields described the incident in a post on Breitbart News
on Thursday and she posted a picture of a bruised arm on Twitter.
"Someone had grabbed me tightly by the arm and yanked me down. I almost fell to the ground, but was able to maintain my balance. Nonetheless, I was shaken," she wrote. "The Washington Post's Ben Terris immediately remarked that it was Trump's campaign manager, Corey Lewandowski, who aggressively tried to pull me to the ground. I quickly turned around and saw Lewandowski and Trump exiting the building together. No apology. No explanation for why he did this."
"Even if Trump was done taking questions, Lewandowski would be out of line," she wrote.
Earlier on Wednesday, Trump spokesperson Hope Hicks released a statement in which she not only doubted Fields' claims but called her credibility into question.
"The accusation, which has only been made in the media and never addressed directly with the campaign, is entirely false," Hicks wrote. "We leave to others whether this is part of a larger pattern of exaggerating incidents, but on multiple occasions she has become part of the news story as opposed to reporting it."