During a speech in New York state's second largest city, Buffalo, Mr Trump referred to the September 11 attacks as "7 Eleven", the name of a chain of convenience stores.
He said: "It's very close to my heart because I was down there, and I watched our police and our firemen down at 7-Eleven, down at the World Trade Center right after it came down, and I saw the greatest people I've ever seen in action."
Mr Trump paid his first visit to the National September 11 Memorial Museum in early April, and the gaffe comes just ahead of the New York primaries
During the Buffalo rally which attracted more than 11,000 supporters, authorities were forced to break up an anti-Trump demonstration.
Mr Trump told supporters 'no New Yorker' could possibly vote for his rival Ted Cruz during Wednesday's primary.
