Service records appear to show the billionaire received a medical deferment, apparently for a bone spur, during the Vietnam War.
Donald Trump received four student deferments and a medical deferment which ruled him out of military service during the Vietnam War, it has been reported.
The Republican candidate has been under intense criticism from party rivals and veterans after he criticised Senator John McCain's war record.
The tycoon, who was enrolled at the New York Military Academy during his high school years, has refused to apologise after suggesting Sen McCain was not a war hero despite being shot down in Vietnam and held captive in a Hanoi prison, where he was tortured over five years.
Records at the National Archives and Records Administration reveal that Mr Trump had received four student deferments and a medical deferment from service by the time his number was drawn on 1 December, 1969, The Smoking Gun reported.
He got his first two students deferments in June 1964 and December 1965, while he was studying at Fordham University in the Bronx, the website said.
He was then reclassified as "available for military service" in November 1966, but that was switched back just weeks later.
The fourth student deferment took place on 16 January, 1968, months before he graduated from the University of Pennsylvania.
Again he was reclassified as available for service on 9 July, but was then reclassified as qualified "only in time of national emergency" after he underwent an Armed Forces Physical Examination according to his records.
Mr Trump has in the past said he received the medical deferment from military service in 1968 because of a bone spur in one of his feet.
He has not commented on the revelations made by The Smoking Gun, but is likely to face more questions after his remarks about Sen McCain.
Mr Trump caused controversy at an Iowa political forum at the weekend, saying of Sen McCain: "He's not a war hero. He was a war hero because he was captured. I like people who weren't captured."
Sen McCain responded to the remarks and called on Mr Trump to apologise to all veterans.
"I think he may owe an apology to the families of those who have sacrificed in conflict and those who have undergone the prison experience in serving our country," he said in an interview on MSNBC.
Supporters in early voting states such as Iowa, New Hampshire and South Carolina have argued the row has been overblown by the media after Mr Trump has enjoyed a rise in polls.
Republican rivals have also criticised the 69-year-old, including Florida Governor Jeb Bush, former Texas Governor Rick Perry and Wisconsin Governor Scott Walker.