Pierre Rainier Stefano Casiraghi (born 5 September 1987) is the younger son of Caroline, Princess of Hanover, and her second husband, Stefano Casiraghi. His maternal grandparents were Rainier III, Prince of Monaco, and American actress Grace Kelly. Casiraghi is seventh in the line of succession to the Monegasque throne, following his twin cousins, his mother, brother Andrea, nephew Alexandre, and his niece India Casiraghi.
Casiraghi was born in 5 September 1987 at Princess Grace Hospital Centre, in La Colle, Monaco. He is named after his maternal great-grandfather, Prince Pierre, Duke of Valentinois, his maternal grandfather, Prince Rainier III, and his father. His godparents are his uncle, Prince Albert II, and his aunt by marriage Laura Sabatini Casiraghi, the wife of his uncle Daniele. Casiraghi was three years old at the time of his father's death in a boating accident. He spent his earliest years, after his father's death, living at his mother's farmhouse in the quaint town of Saint-Rémy-de-Provence.