• He arrived in London on Friday, but will reportedly return to the US after the ceremony for his son's birthday.
The Duke of Sussex has joined other royals at Westminster Abbey for his father King Charles III's Coronation.
Prince Harry appeared relaxed as he arrived at the Abbey and was seen chatting to his cousins Princess Beatrice and Princess Eugenie.
It is thought it is the first time he has seen his family since his controversial memoir, Spare, came out.
He arrived in London on Friday, but will reportedly return to the US after the ceremony for his son's birthday.
Prince Archie has remained in Los Angeles with his mother, the Duchess of Sussex, where he will celebrate his fourth birthday.
A source told the US outlet Page Six that Prince Harry intended to make "every effort to get back in time for Archie's birthday".
As he entered the Abbey, dressed in a morning suit and wearing medals, the prince appeared relaxed and was seen smiling at guests as he walked down the nave of the abbey, before chatting to several family members.
He tapped Princess Beatrice's husband Edoardo Mapelli Mozzi on the back with a smile on his face, before taking his place beside Jack Brooksbank - husband of Princess Eugenie - in the third row. Also in the third row was the Duke of York, Prince Andrew.
He watched the ceremony behind the King's sister Princess Anne and the Duke of Kent, Queen Elizabeth II's first cousin, dressed in Garter robes on the second row.
Ahead of them were the Prince and Princess of Wales and their children Princess Charlotte and Prince Louis, as well as the King's brother the new Duke Edinburgh - Prince Edward - and his wife Sophie.
Previously, Prince Harry had sat in the second row - directly behind the King - at Westminster Abbey for the late Queen Elizabeth's funeral last year.
After landing at London's Heathrow Airport on Friday morning, the prince reportedly stayed at Frogmore Cottage on the grounds of Windsor Castle on Friday night - the residence he and Meghan were asked to vacate by the King.
Last month Buckingham Palace confirmed that Prince Harry would attend the ceremony alone.
However, officials have said Prince Harry and his uncle Prince Andrew will have no formal role in the ceremony, as they are not working members of the Royal Family.
Unlike other members of the family, he is not wearing robes or military uniform - despite the fact that he served in the army for a decade, including two tours of Afghanistan.
Meghan stays in the US
Relations between Prince Harry and other members of the family have been strained since his memoir was published.
The book vividly revealed fall-outs and disagreements with relatives, and he has since spoken of feeling "different" from the rest of his family.
The decision for Meghan to reject the invitation was widely seen as part of these continuing, unresolved family tensions.
And last month it was revealed that the King tried to stop Prince Harry taking legal action against newspapers over alleged phone-hacking.
In a witness statement revealed by court papers, Prince Harry said he was "summoned to Buckingham Palace" and told to drop the cases because of the effect on the family.