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.
She will attend high profile celebrations in central London marking the centenary of the RAF on Tuesday, and a visit by USA president Donald Trump in Windsor on Friday.
Louis' godparents, who will be announced on the day, will take part in the private 40-minute traditional religious ceremony, which begins at 16:00 BST.
The prince will be baptized in the Lily Font, a silver font ordered by Queen Victoria and Prince Albert for the baptism of their daughter Victoria in 1841.
Kate's sister Pippa is expecting her first child.
William was seen giggling as his middle child stared down at the crowds of press, while Kate Middleton held her newborn son in her arms.
Prince Harry, Meghan Markle, Prince Charles and his wife Camilla, and the Middleton's parents and siblings all attended the service.
In 2015, his sister Princess Charlotte was christened at St. Mary Magdalene Church in Sandringham, a royal estate near England's eastern coast.
It's also where the coffin of Diana, Princess of Wales laid for a week before her burial.
Six godparents included the Duchess's cousin, solicitor Lucy Middleton, a handful of the Duke's school friends and reformed "court jester" Guy Pelly.