He or she will accompany William and Kate on their visit to New Zealand and Australia in April to look after their son while they carry out official duties, a spokesman said.
Palace officials would not confirm who the nanny is but the couple may opt to take William's former nanny Jessie Webb, who has been helping the Cambridges with childcare duties on an ad hoc basis.
The baby will be eight months old when he flies Down Under with his parents on his first official overseas tour.
The three-week visit will echo the Prince and Princess of Wales's visit to Australia and New Zealand in 1983 when the couple took William, then just nine months old, with them.
The little Prince will be seen in public at least four times when he embarks and disembarks aircraft.