List of Thai monarchs

The succession of Thai monarchs began with Si Inthrathit at the establishment of the first Thai kingdom in 1238. With brief interruptions, 55 monarchs have ruled over four successive kingdoms, the current monarch being Vajiralongkorn (Rama X) of the Chakri dynasty.

Titles and naming conventions
In the Sukhothai Kingdom, the monarch used the title Pho Khun (พ่อขุน), and monarchs who reigned over the period of decline after Ram Khamhaeng the Great used the title Phraya (พญา).

In the Ayutthaya Kingdom and afterward, thanandon, the system of Thai royal titles, determines the style of the monarch's full regnal name (which includes the title), consisting of two interconnected parts:
 * The first part is the title Phra Bat Somdet Phra Chao Yu Hua (พระบาทสมเด็จพระเจ้าอยู่หัว, 'His Majesty the King'). The title is split by the regnal name, written between Phra and Chao. If the monarch was not formally crowned, Phra Bat is omitted. If the monarch ruled as a tributary or was usurped, Somdet may be omitted.
 * Sometimes, Chao Yu Hua is fully omitted. Other times, only Yu Hua is omitted, in which case Chao follows the remaining components of the title and precedes the regnal name.
 * The second part is the regnal name, of which only a portion may be used to commonly refer to the monarch. It may differ from their birth name, their name as uparaja (viceroy), or their posthumous name and/or historical style. This list refers to monarchs by the names most often used by traditional historians.

Western nations referred to the monarch as the "King of Siam" (Rex Siamensium), regardless of Thai titles, since the initiation of relations in the 16th century. Mongkut (Rama IV) was the first monarch to adopt the title when the name Siam was first used in an international treaty. When the kingdom's name was changed to Thailand, the monarch's Western title changed accordingly.

Sukhothai Kingdom (1238–1438)
Tai peoples migrated into mainland Southeast Asia in the 8th–10th centuries. In the years after, Northern Thai groups established mueang that evolved into larger states, such as Ngoenyang. However, it was not until the decline of the Khmer Empire in the 13th century that a Central Thai kingdom politically and culturally related to modern Thailand was first founded.

Phra Ruang dynasty (1238–1438)
The Phra Ruang dynasty was the only royal lineage that ruled over the Sukhothai Kingdom, the first Central Thai state. Established by Si Inthrathit in 1238, who declared independence from the Khmer Empire, the dynasty laid the foundations for Thai society. Under Ram Khamhaeng the Great, the initial Thai script was invented and Therāvada Buddhism was established as the state religion.

The dynasty is named after the Traiphum Phra Ruang, a Buddhist cosmology book written by Maha Thammaracha I. During his reign, the kingdom was invaded by Ayutthaya, a neighboring Thai state, becoming a tributary during the reign of Maha Thammaracha II.

In the Sukhothai Kingdom, the monarch ruled from the city of Sukhothai, while the heir presumptive would occasionally be named uparaja, or viceroy, and ruled in Si Satchanalai. In 1438, Ayutthaya annexed Sukhothai at the death of Maha Thammaracha IV when Borommarachathirat II of Ayutthaya named his son Prince Ramesuan uparaja. In Ayutthaya, the tradition would evolve into the Front Palace system.

Ayutthaya Kingdom (1351–1767)
The Aytthaya Kingdom was a result of a unification between two states: The Kingdom of Suphannaphum (Suphanburi) and The Kingdom of Lavo. Though the Kingdom of Lavo was originally a Mon kingdom, the migration of the Tai peoples into the Chao Phraya basin replaced the original Mons, consequently becoming the governors of these regions. This resulted in the capital moving from Lavapura (Lopburi) and Suphannaphum (Suphanburi) to Ayodhya (Old Ayutthaya), a new capital. Later on, the Kingdom of Nakhon si Thammarat came under the influence of Ayutthaya after seceding from Sukhothai, and Sukhothai losing influence and coming under Ayutthaya.

This conflict would last long into the history of Ayutthaya Kingdom where families from the four major regional kingdoms vie over the throne of the kingdom. These kingdoms are: Kingdom of Nakhon Si Thammarat, Kingdom of Sukhothai, Kingdom of Lavo, and Kingdom of Suphannaphum. The Kingdom of Sukhothai was integrated into the Kingdom of Ayutthaya along with its noble famlies.