Harry told reporters he knew Markle was "the one" from "the very first time we met."
The prince, 33, and Ms. Markle, 36, will marry in March as no previous royal engagement has lasted longer than five months.
Prince Harry and Ms. Markle are expected to be known as the duke and duchess of Sussex after they are married, according to the British news media, although she may be known informally as Princess Meghan.
The couple will live in Nottingham Cottage at Kensington Palace.
The ring was designed by Harry with a gold band and features a large diamond from Botswana and two smaller outer diamonds from the personal collection of Harry's late mother, Diana, Princess of Wales.
Botswana holds special significance for the couple, who have spent time together there over the past year and a half.
The jewel was made by Cleave and Company, Court Jewellers and Medallists to Her Majesty The Queen.
