Archbishop of Canterbury Justin Welby, the spiritual leader of the world's Anglicans, will lead the service in the Chapel Royal at St James's Palace.
Following the 40-minute service, The Duke and Duchess of Cambridge gave a private tea at Clarence House.
This isn't the first time Prince George's little sister has worn the brand - Pepa & Co are same company that made the together page boy and flower girl outfits for aunty Pippa Middleton's wedding to James Mathews in 2017.
The godparents included Hannah Carter, a school friend of the Duchess, and Lady Laura Meade, who is married to Prince William's friend and Princess Charlotte's godfather, James. Kate looked radiant as she pushed the vintage Millson pram that her grandmother-in-law the Queen used in the 1950s for her sons Prince Andrew and Prince Edward.
Princess Charlotte is proving, yet again, that she's nailed the royal wave technique.
The Duke of Edinburgh, 97, has retired from public duties and has been enjoying his free time away from London.
The duchess, who was wearing Alexander McQueen, added, with a smile: "I hope he stays like this".
His elder brother George was also christened in the Chapel Royal, but sister Charlotte was baptised at the Church of St Mary Magdalene in Sandringham.
Kensington Palace said William's father Prince Charles and his wife Camilla would be present along with younger brother Prince Harry and his wife Meghan. Cheeky Charlotte glared intensely at the press as she told them: "You're not coming".
Prince George, who turns five later this month, looked quite nonplussed despite the significance of the occasion.
If some of the name sounds familiar, it's because several members of Prince Louis' list are related to godparents chosen for the other tiny royals.