The duchess gave birth Monday morning, and emerged from St. Mary's Hospital in London about seven hours later alongside her husband, holding the baby wrapped in a white blanket. The trio posed for photographers before going back inside.
Prince Louis is fifth in line to the British throne, after grandfather Prince Charles, father William, Prince George and sister Princess Charlotte. He is Queen Elizabeth II's sixth great-grandchild.
The British royal family is set for a big year, with Prince William's brother Prince Harry set to marry American actress Meghan Markle on May 19 at Windsor Castle, outside London.