Prince Louis of Cambridge is being christened in the Chapel Royal, watched by his older brother Prince George and big sister Princess Charlotte.
Proud parents the Duke and Duchess of Cambridge made their way into the service in London with their three children.
George, four, and three-year-old Charlotte held William's hand, as a sleeping Louis was carried into the chapel by Kate.
Eleven-week-old Louis was dressed in the cream, frilly lace royal christening gown - a replica of the intricate robe made for Queen Victoria's eldest daughter in 1841.
The Archbishop of Canterbury welcomed the royals, bending down to greet George and Charlotte, who looked happy and content, at the entrance to the chapel.
As Kate chatted to the archbishop while holding Louis, she could be heard describing her sleeping son as "very relaxed and peaceful".
The duchess added, with a smile: "I hope he stays like this."
Louis's aunt and uncle the Duke and Duchess of Sussex were among the guests in the chapel at St James's Palace in London.
Meghan - who is attending her first royal christening - looked stylish, dressed in an olive-green Ralph Lauren outfit and Stephen Jones hat.
The Queen and the Duke of Edinburgh were missing from the small family affair.
The decision is understood not to have been taken on health grounds, and to have been mutually agreed by the Queen and the Cambridges some time ago.
The Queen, 92, has a busy week ahead, with high-profile celebrations in central London marking the centenary of the RAF on Tuesday, and a visit by US President Donald Trump in Windsor on Friday.
The Duke of Edinburgh, 97, has retired from public duties.
It is the first time the Cambridge family have been seen together as a family of five.
Louis has not been seen, other than in photographs released by Kensington Palace, since his debut outside the Lindo Wing on the day he was born.
The prince's six godparents were among the guests including the duke's close friend Guy Pelly - who was once dubbed William and Harry's "court jester" for his wild ways.
Louis's other godparents are: Nicholas van Cutsem - one of the van Custem brothers, who are long-time family friends of William and Harry; Harry Aubrey-Fletcher, who went to Ludgrove School and Eton with William; Lady Laura Meade, who is married to William's close friend James Meade; Kate's Marlborough schoolfriend Hannah Carter (nee Gillingham); and the duchess's cousin Lucy Middleton.
There for the ceremony were Louis's grandfather the Prince of Wales, step-grandmother the Duchess of Cornwall and maternal grandparents Carole and Michael Middleton.
Kate's brother James Middleton, and Kate's pregnant sister Pippa with her husband James Matthews - the brother of former Made In Chelsea star Spencer Matthews - were also among the guests.
Prince Louis Arthur Charles of Cambridge, fifth in line to the throne, was born on St George's Day, April 23, at the private Lindo Wing of St Mary's Hospital in Paddington, London, weighing 8lb 7oz.