His  Majesty  King  Bhumibol  Adulyadej  was born in Cambridge,  Massachusetts,  the United States of America, on Monday the  5th  of  December 1927, being the youngest son of H.R.H. Prince Mahidol of Songkla and Mom Sangwalya Mahidol (H.R.H. the Princess Mother) and the direct grandson of His Majesty King Chulalongkorn (Rama V).

He has one elder sister, H.R.H. Princess Galayani Vadhana  and  one elder brother, His Majesty King Ananda Mahidol - Rama VIII.

After graduating from Harvard University's Medical School, Prince Mahidol came  back to Thailand and passed away when His Majesty was not yet two years old. The family then moved to Switzerland where The King continued his secondary education at the  Ecole  Nouvelle  de  la Suisse Romande, Chailly sur Lausanne, and received the  Bachelieres Lettres diploma from the Gymnase Classique Cantonal of Lausanne.

In 1935, after King Prajadhipok (Rama VII) abdicated, his  elder brother, Prince Ananda Mahidol, was proclaimed by the government as the eighth King (Rama VIII) of the Chakri Dynasty. The  new King at the age of 10,  remained in Switzerland persuing his studies, cut off from his homeland by World War II. In 1946 King Rama VIII died while on a visit to Thailand and Prince  Bhumibol  Adulyadej, then 19, was chosen to succeed his  brother. 

His  Majesty  decided to go back to Switzerland to complete his education. In 1950, he returned to Thailand  for  the  Coronation  Ceremony and  went  back to Switzerland  for  another  period  of studies  before returning to Thailand in 1951.

His Majesty married Mom Rajawongse Sirikit Kittiyakara (Her majesty  Queen Sirikit) on the 28th of April 1950. 

Their Majesties have four children namely :

1. H.R.H. Princess Ubol Ratana, born on the 5th of April 1951.
2. H.R.H. Crown Prince Maha Vajiralongkorn, born on the 28th of July 1952.
3. H.R.H.Princess Maha Chakri Sirindhorn, born on the 2nd of April 1955.
4. H.R.H.Princess Chulabhorn, born on the 4th of July 1957.