The Sukhothai Kingdom was a post-classical Siamese kingdom (maṇḍala) in Mainland Southeast Asia surrounding the ancient capital city of Sukhothai in present-day north-central Thailand. It evolved from a trading hub to a city-state in 1127 and emerged into the kingdom by Si Inthrathit in 1238. Sukhothai existed as an independent polity until 1438 when it fell under the influence of the neighboring Ayutthaya after the death of Borommapan (Maha Thammaracha IV).

Sukhothai was originally a trade center in Lavo—itself under the suzerainty of the Khmer Empire from 946–1052—when Central Thai people led by Pho Khun Bang Klang Hao, a local leader, revolted and gained their independence. Bang Klang Hao took the regnal name of Si Inthrathit and became the first monarch of the Phra Ruang dynasty.

The kingdom was centralized and expanded to its greatest extent during the reign of Ram Khamhaeng the Great (1279–1298), who some historians considered to have introduced Theravada Buddhism and the initial Thai script to the kingdom. Ram Khamhaeng also initiated relations with Yuan China, through which the kingdom developed the techniques to produce and export ceramics like sangkhalok ware.

After the reign of Ram Khamhaeng, the kingdom fell into decline. In 1349, during the reign of Li Thai (Maha Thammaracha I), Sukhothai was invaded by the Ayutthaya Kingdom, a neighboring Thai polity. It remained a tributary state of Ayutthaya until it was annexed by the kingdom in 1438 after the death of Borommapan. Despite this, the Sukhothai nobility continued to influence the Ayutthaya monarchy in centuries after through the Sukhothai dynasty.

Sukhothai is traditionally known as "the first Thai kingdom" in Thai historiography, but current historical consensus agrees that the history of the Thai people began much earlier. The ruins of the kingdom's capital, now 12 km (7.5 mi) outside the modern town of Sukhothai Thani in Sukhothai Province, are preserved as the Sukhothai Historical Park and have been designated a UNESCO World Heritage Site.

The English term Sukhothai (Thai: สุโขทัย) is the romanization of the Thai word per the Royal Thai General System of Transcription. The Thai word for the historical country was a transliteration of the Khmer spelling, rendered in English as Sukhodaya (Khmer: សុខោទ័យ). The Khmer term is itself derived from the Sanskrit sukha (Sanskrit: सुख, 'lasting happiness') and udaya (Sanskrit: उदय, 'rise' or 'emergence'). Together, the phrase can be interpreted as meaning "dawn of happiness".

ฺBorihan Thepthani claimed the city of Sukhothai was founded in 494 CE, followed by its twin city of Chaliang (เชลียง). In contrast, the Legend of the Arhat (Tamnan Nithan Phra Arahant; ตำนานนิทานพระอรหันต์) says Sukhothai was founded in 679 CE by Indrajayadhirāja (อินทราไชยธิราช), who was from the city of Nakhon Luang (นครหลวง; potentially Lavapura); however, he, in 687, was deposed by Balidhiraja, the elder son of Kalavarnadisharaja of Lavo. The following rulers after Balidhiraja remain unknown. During the decline of Dvaravati's Lavo in the late 8th century, it was expected that the city would be abandoned for a few centuries due to the invasion by the legendary Khom kingdom of Suvarṇakōmakam (สุวรรณโคมคำ) in the present-day Chiang Saen and was then re-established in 957 by Abhayakāminī who evacuated from Haripuñjaya after the capital was seized by another Khom polity, Umoṅkaselā (อุโมงคเสลา) in modern Fang.

The chiefdom of Sukhothai declared independence from Umoṅkaselā in 1017 during the reign of Arunaraja. Through its control of trade routes linking Mon city-states in the west, Tai kingdoms in the north, and Xiān polities in the lower Chao Phraya River basin in the south, Sukhothai is thought to have emerged as a regional logistics hub and to have developed into a city-state level by no later than 1127, a condition that appears to have persisted until the formal establishment of the Sukhothai Kingdom in 1238.