| Crown Prince of Thailand | |
|---|---|
| สยามมกุฎราชกุมาร | |
 | |
Incumbent since 13 October 2016Vacant | |
| Style | His Royal Highness |
| Type | Heir apparent |
| Member of | Chakri dynasty |
| Appointer | Monarch |
| Term length | Life tenure or until accession as Sovereign |
| Precursor | Viceroy of Siam |
| Inaugural holder | Vajirunhis |
| Formation | 14 January 1886 |
The Crown Prince of Thailand (or Siam; Thai: สยามมกุฎราชกุมาร; RTGS: Sayammakutratchakuman; lit. the royal son of Siam) is a title held by the heir apparent to the Thai throne. It was created by King Chulalongkorn (Rama V) in 1886, for his son Prince Maha Vajirunhis, the king's eldest son by a royal wife Queen Savang Vadhana. Prior to this, the Siamese throne did not have a law or formal system regulating royal succession. In 1688, King Phetracha of Ayutthaya created the title of Front Palace, which by the Rattanakosin period had become the main title granted to the heir presumptive to the throne. However, few Front Palaces succeeded to the throne this way, with the exception of King Phutthaloetla Naphalai (Rama II) in 1809. After the death of Bowon Wichaichan in 1885, the title of Front Palace was abolished and replaced with the title of Crown Prince, who became heir apparent to the throne.
In 1924, King Vajiravudh (Rama VI) promulgated the 1924 Palace Law of Succession to regulate the succession. This law essentially barred females, children of commoner wives or children of foreign wives to the throne. It also re-affirmed agnatic primogeniture, or succession through the male line by seniority. This law also affected the individuals who could become Crown Prince. Since its creation, three Princes have been raised to this title, and two have succeeded to the throne.
The title in Thai, Sayammakutratchakuman, comes from conjugation of the words Sayam (Siam), Sanskrit makut (meaning "crown"), ratcha (from Sanskrit rāj), and kuman (from Sanskrit kumār, meaning "son").