Lady Diana Spencer was never officially "Princess Diana," despite her 1981 marriage to Prince Charles of Wales. BBC News reported that she held the title of Princess of Wales while wed to Charles. After their divorce, she became Diana, Princess of Wales.
Prince Harry's grandmother, Queen Elizabeth II, is expected to bestow a title on Harry on the morning of his nuptials, following royal tradition. The Guardian reported that he will likely become the Duke of Sussex, making Markle a duchess.
Harry’s older brother, Prince William, was made the Duke of Cambridge in 2011, when he wed Catherine “Kate” Middleton, making his wife the Duchess of Cambridge. Middleton is not officially called “Princess Kate.”
British royal family officials announced Monday that Prince Harry, 33, and Markle, 36, became engaged in London in November. The couple will wed next spring and plan to live at Kensington Palace's Nottingham Cottage.
Markle, who has portrayed Rachel Zane on the USA series "Suits" since 2011, started dating Harry in July 2016.