This also marked a rare public appearance in general for Louis, who remained at home at about six weeks old during last year's event.
See photos from the Trooping the Colour parade:
William's brother Prince Harry and wife Meghan Markle also attended the event and appeared with the family on the balcony. It marked the first public appearance for the Duchess of Sussex since she gave birth to the couple's first child, son Archie Harrison Mountbatten-Windsor, one month ago. Days after he was born, Meghan and Harry showed him off in a brief and small press photo call at Windsor Castle.
Other royals spotted on the Buckingham Palace balcony included William and Harry's father Prince Charles and his wife Camilla, Duchess of Cornwall, Anne, Princess Royal, Princess Beatrice and sister Princess Eugenie, their dad Prince Andrew, Duke of York, plus William and Harry's cousin Peter Phillips and wife Autumn Phillips' daughter Isla Phillips and Savannah Phillips, who last year made headlines when she playfully teased George at Trooping the Color. The Cambridge children stood far away from the Phililips girls at Saturday's event.
Not in attendance: The queen's husband Prince Philip, who rarely appears at royal events these days, as he is retired. He is set to turn 98 on Monday.