The US president’s boast came as he criticised the response to the massacre by authorities in Florida.
It was revealed that Scot Peterson, the armed officer assigned to Marjory Stoneman Douglas High School, admitted he never entered the building to confront the attacker.
Mr Trump told a group of state governors on Monday evening: “I really believe I’d run in there even if I didn’t have a weapon.
“I think most people in this room would have done that too…You never know until you’re tested.”
His comments come after Florida governor Rick Scott launched an inquiry into the local police response to the Valentine’s Day massacre which saw 17 students and staff members murdered.
In a statement, Mr Peterson’s lawyer said: “Let there be no mistake, Mr Peterson wishes that he could have prevented the untimely passing of the seventeen victims on that day, and his heart goes out to the families of the victims in their time of need.
“However, the allegations that Mr Peterson was a coward and that his performance, under the circumstances, failed to meet the standards of police officers are patently untrue.
“Mr Peterson is confident that his actions on that day were appropriate under the circumstances and that the video (together with the eye-witness testimony of those on the scene) will exonerate him of any sub-par performance.”
Mr Trump’s claim provoked a furious backlash online. Jon Cooper, chairman of the US Democratic Coalition, held a Twitter poll in which 98 per cent of voters called Mr Trump a “lying sniveling coward” over a “daring, courageous hero”.
Shooting survivor Rachel Pilicer wrote: “The fact that Trump is using the deaths at my school[,] ‘I’d run in there, even without a weapon’, to make himself look heroic is absolutely disgusting.”
Meanwhile Family Guy creator Seth Macfarlance shared Mr Trump’s quote alongside a Gif of the president cowering next to a startled bald eagle:
The debate over how to protect children from gun violence in the US intensified this week as Mr Trump’s daughter, Ivanka, admitted that she did not know whether her father’s proposal to arm teachers would work.
Her lukewarm response to the US president’s plan raised new questions over the mixed messages from the White House in the aftermath of the school shooting.
In an interview in South Korea, where she was attending the closing ceremony for the Winter Olympics, Ms Trump answered “I don’t know,” when asked if she would be comfortable knowing that a teacher educating one of her three children was in possession of a gun in the classroom.
“Obviously, there would have to be an incredibly high standard for who would be able to bear arms in our school. But I think there is no one solution for creating safety,” she added.
Mr Trump tweeted the proposal in the immediate aftermath of the Florida massacre. He suggested new legislation mandating that 20 per cent of teachers should carry guns in the classroom, rewarded with a yearly cash bonus.
Powerful lobby group the US National Rifle Association has said that it does not support a gun ban following the Florida attack.