The Archbishop of Canterbury, the Most Rev Justin Welby, is baptising him in front of close family and friends in the Chapel Royal at St James's Palace.
The three-month-old, who will one day be king, made only his second appearance in public for the christening.
His only previous public outing was when William and Kate left the Lindo Wing of St Mary's Hospital in central London the day after he was born on July 22.
On that occasion, they proudly showed him off to the world's media and could not contain their joy as he slept peacefully.
Since then his father has revealed that George has a good pair of lungs so today's event may be a noisy affair.
His private christening, like his birth, has attracted interest from across the globe.
A picture will be released tomorrow to mark the baptism, showing four generations of the Royal Family together.
The photograph will feature the Queen with three future monarchs - the Prince of Wales, William and George.
It will echo the picture taken in July 1894 during the christening of the future Edward VIII, showing the royal baby with his father, later George V, grandfather, the future Edward VII, and great grandmother, Queen Victoria.
When the Queen's car arrived, nearby doors in the palace opened and young George could be heard gurgling.
His grandfather Charles and stepmother Camilla walked forward to meet the Queen, who was already being greeted by the Archbishop and the Bishop of London Richard Chartres.
The doting grandfather was followed by William and Kate, with the duke holding his son in front of him and jiggling him up and down as he walked.
He was joined by his brother Harry, who greeted his grandfather the Duke of Edinburgh as William told everyone: "He's all ready", adding: "So far so good."
George, with rosy cheeks and darkish hair, stared curiously at his relatives as they chatted and was on his best behaviour.
He was wearing in a replica of the intricate lace and satin christening gown made for Queen Victoria's eldest daughter in 1841, which almost touched the ground.
The royal party chatted for a few more minutes and Kate walked over to greet Philip, kissing him on both cheeks before curtseying quickly.