Louis is fifth in the line of succession to the British throne after his grandfather Prince Charles, his father William and his older siblings George and Charlotte.
Also attending are Kate's parents, her brother and sister and Louis's six godparents, who are close friends or relatives of William and Kate.More news: Croatia star Modric expects a tough game against a 'strong' England
The 11-week-old Prince wore the Honiton lace and white christening robe that was also worn by Prince George and Princess Charlotte.
The event is going to be attended by grandparents The Prince of Wales and The Duchess of Cornwall, as well as Prince Harry and Meghan, The Duke and Duchess of Sussex.
While this is a meaningful day for Will and Kate's third child, July 9 already marks a memorable date for the Royal clan: It was on this day in 1947 that Queen Elizabeth and Prince Philip announced their engagement, and posed for photos at Buckingham Palace.
Prince Louis has been christened in a private ceremony.More news: Real Madrid and Juventus close to agreeing Cristiano Ronaldo deal
Uncle Harry has also promised to build the little prince a library in memory of his late mother, Princess Diana, according to the Sun U.K. The robe, which was handmade by Angela Kelly, the Queen's personal dressmaker, is a replica of the original christening gown designed for Queen Victoria's eldest daughter.
Guy Pelly: The son of wealthy Kent landowners, Mr Pelly used to help run several London nightclubs. He has been banned from driving three times, including twice for drink-driving and once for speeding.
"Robinson Crusoe was William's favourite book, but Harry loved all things A.A. Milne", the source added. She is the wife of Robert Carter, who belongs to a family of wealthy landowners.
Lucy Middleton: She is the Duchess of Cambridge's cousin and a solicitor.More news: Johnny Depp Sued By Crew Member For Allegedly Punching Him