The Duchess of Cambridge, as she has been formally known since her marriage two years ago to Prince William, used a 4 gallon (15 liter) bottle of Moet and Chandon Champagne to do the job - it was so heavy she couldn't swing it herself, but she cut a ribbon that launched the bottle toward the hull, where it smashed as crowds cheered.
"I name this ship Royal Princess, may God bless her and all who sail in her," said the duchess, whose baby bump was clearly outlined against her black and white print coat.
She seemed the picture of good health as she chatted with others on the VIP podium.
The duchess's bottle-smashing performance was the highlight of a gala ceremony that included a formal blessing and a prayer from the Bishop of Winchester and a performance by pop star Natasha Bedingfield.
The Duchess of Cambridge has been officially designated the godmother of the new ship, a symbolic title dating back to an earlier nautical era.
She and William are expecting their first child in mid-July. The baby will become third in line for the throne behind William and his father, Prince Charles.
William's mother, the late Princess Diana, performed a similar cruise ship christening on the same spot nearly three decades ago.
The ceremony in Southampton on Thursday included a brief tour of the ship for the duchess.
It is expected to be her final solo engagement before the birth, although she is likely to join other senior royals at the upcoming Trooping the Color ceremony.
The duchess's visit highlighted the importance of the new 3,600-passenger Royal Princess, which is scheduled to begin cruising the Mediterranean this summer.