Mother-of-two Ciara Harmon from Co Kildare helped the popular tourist attraction reach the milestone after purchasing a family ticket along with her husband John and children Nicole, who is sixteen, and 12-year-old Aaron.
Titanic Belfast first opened in April 2012, marking the 100th anniversary of the ship's tragic maiden voyage.
The attraction has welcomed visitors from over 145 countries, ranging from the US to China.
Ciara Harmon was greeted at Titanic Belfast on Tuesday by CEO Tim Husbands and The Lord Mayor of Belfast, Councillor Máirtín Ó Muilleoir.
Speaking about her special visit, Ciara said: "It was a bit of a shock when I got the call from Titanic Belfast. We had just planned to come up to the city for the day and instead it has turned out to be an occasion we will never forget.
"It's amazing to see how much Belfast has transformed; it's a wonderful city and we are pleased to be part of this great occasion for Titanic Belfast.
Titanic Belfast CEO, Tim Husbands, added: "Once again, and ahead of time, our own expectations have been exceeded, and we are very proud to host Ciara Harmon and her family, along with all of our guests on a momentous occasion for Titanic Belfast.
"One million visitors have come from 145 countries to walk through the world's largest Titanic exhibition. Those numbers are testament to the appeal of Titanic, and the appeal of Belfast as a new, vibrant, confident tourist destination."