Willem-Alexander has become the first Dutch king in 123 years, following the abdication of his mother Queen Beatrix at the Royal Palace in Amsterdam.
An emotional Beatrix, 75, ended her 33-year-reign signing the official act of abdication in a nationally televised ceremony on her official celebration day, the Koninginnedag or Queen's Day national holiday.
A jubilant 20,000-strong crowd bedecked in orange cheered the royal handover outside the palace in central Amsterdam and millions more watched the ceremony on television.
The former Queen later appeared on a balcony to introduce her eldest son as the new King.
"I am happy and grateful to introduce to you your new king, Willem-Alexander," she told the crowd, almost in tears.
Beatrix then left the balcony leaving space for Willem-Alexander, his wife and three daughters - the children in matching yellow dresses and headbands - who waved to the crowd.
Beatrix will be now on referred to as Princess Beatrix. Willem-Alexander, 46, Beatrix eldest son, is the first Dutch king since Willem III died in 1890.
Willem-Alexander is married to Argentine-born Máxima Zorreguieta Cerruti, 41, a former New York investment banker, who becomes Queen Maxima While their eldest daughter, Catharina-Amalia, becomes Princess Orange and first in line to the throne.
"Now that my oldest son is to take over this fine and responsible job tomorrow, it is my deep wish that the new royal couple will feel themselves supported by your loving trust," the popular monarch said in a nationally televised address.
"I am convinced that Willem-Alexander will apply himself with true devotion for everything a good king is obliged to do."
Known in his beer-drinking, student years as "Prince Pilsner", Willem-Alexander is expected to keep a less formal profile than his mother and has already made clear he would not insist on being called "majesty".