The set of 12 photographs were taken at the wedding reception in Buckingham Palace on 29 July 1981, which followed the ceremony at St Paul's Cathedral.
Six of the images show the princess with members of her family and the wedding party, including the Queen.
The photos come from the collection of an assistant to the English photographer Patrick Lichfield.
Lord Lichfield, the Queen's first cousin once removed, was the only photographer allowed to take informal images of the royal family and their guests when they returned to Buckingham Palace.
Some of the images show Princess Diana holding the five-year-old Clementine Hambro, her youngest bridesmaid.
Another shows the royal couple on the Buckingham Palace balcony, overlooking the crowds gathered below.
The collection, due to be auctioned in America next month, is accompanied by a letter of provenance from the assistant to Lord Lichfield, which identifies the people in each photograph and offers further details.
The wedding of Prince Charles to Lady Diana Spencer attracted an estimated global TV audience of 750 million - the most popular programme ever broadcast.
The couple divorced in August 1996 and Princess Diana was killed in a car crash in Paris the year after.
Lord Lichfield died in 2005 at the age of 66 after suffering a stroke.
The Remarkable Rarities live auction event from RR Auction will be held in Boston, Massachusetts, on September 24, 2015.
Auction spokesman Bobby Livingston said: "These never-before-seen photographs capture the candid and unrehearsed moments of the fairytale wedding."
Photographs taken behind the scenes at the royal wedding of Prince Charles and Diana are to be auctioned.