Thailand occupies 513,120 square kilometers on mainland Southeast Asia and extends southward onto the Malay Peninsula. The country shares land borders with Myanmar to the west and northwest, Laos to the north and northeast, Cambodia to the southeast, and Malaysia to the south. Two major bodies of water define Thailand's maritime boundaries: the Gulf of Thailand to the east and south, and the Andaman Sea to the southwest. The coastline measures approximately 3,219 kilometers. The nation's geography divides into four distinct regions: the mountainous north, the Khorat Plateau in the northeast, the Central Plains, and the southern peninsula. Elevation ranges from sea level to 2,565 meters at Doi Inthanon, the country's highest point. Thailand sits entirely within the tropical zone between approximately 5 and 21 degrees north latitude.
The northern region contains Thailand's most prominent mountain ranges, which form part of the larger system extending from the Himalayas through Myanmar. The Thanon Thong Chai Range runs along the Myanmar border in the northwest. The Phi Pan Nam Range extends through the interior of northern Thailand. The Phetchabun Range forms the western edge of the Khorat Plateau and runs roughly north-south through the center of the country. The Dong Phaya Yen Mountains connect the northern highlands to the Khorat Plateau. These ranges consist primarily of granite and limestone formations dating to the Paleozoic and Mesozoic eras. Forests covering these mountains transition from mixed deciduous at lower elevations to evergreen above approximately 1,000 meters, with pine forests at the highest elevations. Average annual rainfall in the northern mountains exceeds 1,500 millimeters, with the southwest monsoon delivering most precipitation between May and October.
Doi Inthanon rises 2,565 meters in Chom Thong District of Chiang Mai Province, making it the highest point in Thailand. The mountain sits within Doi Inthanon National Park, which covers 482 square kilometers established in 1972. The peak was formerly called Doi Luang or Doi Ang Ka before being renamed to honor King Inthawichayanon, the last king of Chiang Mai, whose ashes are interred on the mountain following his death in 1897. The summit supports montane evergreen forest, a rare ecosystem in Thailand that exists only above 1,800 meters. Temperatures at the peak can drop below freezing during December and January. The mountain serves as the watershed source for the Mae Chaem, Mae Khan, Mae Klang, and Mae Pan rivers. More than 360 bird species have been recorded in the park, including several species endemic to the region such as the green-tailed sunbird. The mountain attracts approximately 500,000 visitors annually, with most arriving between November and February during Thailand's cool season.
Doi Suthep reaches 1,676 meters northwest of Chiang Mai city and forms part of the Suthep-Pui mountain range. The mountain gives its name to Doi Suthep-Pui National Park, which encompasses 261 square kilometers established in 1981. Wat Phra That Doi Suthep, built in 1383 during the reign of King Keu Naone of the Lanna Kingdom, sits at 1,073 meters elevation on the mountain's eastern slope. A paved road completed in 1935 connects the temple to Chiang Mai, covering approximately 15 kilometers with numerous switchbacks. The Hmong village of Doi Pui on the northwestern slope sits at approximately 1,300 meters elevation and has been continuously inhabited since the late 19th century. The mountain's forests contain at least 2,000 plant species, including numerous orchid varieties. Wild mammals documented on Doi Suthep include Asiatic black bears, sambar deer, barking deer, and wild boar. The mountain functions as Chiang Mai's primary watershed, feeding several streams that supply the city's water system.
The Dawna Range runs approximately 350 kilometers along Thailand's western border with Myanmar from Mae Hong Son Province southward through Tak and into Kanchanaburi Province. Peaks in this range generally reach between 1,200 and 1,800 meters. The range forms a substantial barrier that historically limited movement between Thailand and Myanmar. Dense deciduous forests cover much of the range, with teak being historically significant until commercial logging ended in 1989. The range's western slopes drain into the Salween River system in Myanmar, while eastern slopes feed tributaries of the Ping and Kwai rivers. Several wildlife sanctuaries protect portions of this range, including Thung Yai Naresuan and Huai Kha Khaeng, which together form a UNESCO World Heritage Site covering 622,200 hectares designated in 1991. These contiguous protected areas contain one of Southeast Asia's largest populations of wild Asian elephants, estimated at over 250 individuals. Tigers, leopards, and Asian wild dogs inhabit the forests, though exact population figures remain uncertain.
The Tenasserim Hills extend approximately 1,700 kilometers from southern Myanmar through the Kra Isthmus into peninsular Thailand and Malaysia. In Thailand, this range runs along the western side of the southern peninsula, forming the spine between the Andaman Sea coast and the interior. Peaks in the Thai portion typically range from 1,000 to 1,500 meters, with some exceeding 1,800 meters in Ranong Province. The range creates a rain shadow effect, with western slopes facing the Andaman Sea receiving over 4,000 millimeters of annual rainfall in some locations, while eastern areas may receive less than 2,000 millimeters. The mountains contain extensive limestone formations with numerous caves. The range's forests represent some of Thailand's wettest and most biodiverse ecosystems, classified as tropical rainforest with year-round moisture. The Tenasserim Hills historically served as a refuge for various ethnic groups and as a corridor for wildlife movement between Myanmar and Malaysia.
Khao Luang rises 1,835 meters in Nakhon Si Thammarat Province on the eastern side of the southern peninsula, making it the highest peak in southern Thailand. The mountain sits within Khao Luang National Park, which covers 570 square kilometers established in 1974. The peak receives exceptionally high rainfall exceeding 3,000 millimeters annually from both southwest and northeast monsoons, maintaining rainforest conditions year-round. The mountain serves as the primary water source for Nakhon Si Thammarat Province, with at least 13 rivers originating on its slopes. More than 300 orchid species grow on Khao Luang, including rare varieties found nowhere else. The mountain's forests contain important populations of dusky langurs, white-handed gibbons, and Malaysian sun bears. Local communities historically collected sap from dipterocarp trees on the mountain's slopes for export as trade goods. The summit area experiences fog and cloud cover on most days throughout the year.
The Khorat Plateau occupies approximately 155,000 square kilometers in northeastern Thailand, comprising nearly one-third of the country's land area. The plateau averages 200 meters elevation, with the terrain gently undulating rather than flat. The Phetchabun Range forms the western boundary, the Dong Phaya Yen Mountains mark the southern edge, and escarpments rise along the northern and eastern boundaries where the plateau meets the Mekong River valley. The plateau's bedrock consists primarily of sandstone and siltstone from the Mesozoic era, forming relatively infertile red and gray soils. Annual rainfall averages 1,200-1,400 millimeters, substantially less than most other regions of Thailand, and droughts occur regularly. The Mun and Chi rivers form the plateau's primary drainage systems, both flowing eastward to join the Mekong River. Salt deposits occur naturally in several areas of the plateau, particularly in the Maha Sarakham and Roi Et provinces, where groundwater dissolves underground salt layers and brings salinity to the surface. The plateau's vegetation historically consisted of dry deciduous dipterocarp forest, though agricultural conversion has removed most original forest cover.
The Central Plains extend approximately 400 kilometers from north to south and reach up to 200 kilometers in width, forming Thailand's most productive agricultural region. The plains slope gently from approximately 150 meters elevation in the north near Nakhon Sawan to sea level at the Gulf of Thailand. The Chao Phraya River, formed by the confluence of the Ping, Wang, Yom, and Nan rivers at Nakhon Sawan, flows through the center of the plains for approximately 370 kilometers before reaching the gulf. The river's gradient through the plains measures only about 0.04%, creating slow-moving water that historically deposited sediments across the floodplain during annual monsoon floods. These alluvial deposits over millennia created soils up to 10 meters deep in some areas, consisting of fine clay and silt. The plains receive 1,000-1,200 millimeters of rainfall annually, concentrated between May and October. Bangkok sits on the lowest portion of the plains where the Chao Phraya River branches into multiple distributaries before reaching the gulf. The city's elevation ranges from 1.5 meters above sea level in most areas to below sea level in some districts, requiring extensive flood control infrastructure.
The Chao Phraya River system drains 157,924 square kilometers, approximately 30% of Thailand's land area. The Ping River, the longest tributary, originates in the mountains of Chiang Mai Province and flows approximately 658 kilometers before joining the Chao Phraya at Nakhon Sawan. The Nan River originates in Phayao Province and travels approximately 627 kilometers. The Wang and Yom rivers contribute smaller but significant flows. From the confluence at Nakhon Sawan, the main channel flows 372 kilometers to the Gulf of Thailand. Average discharge at Nakhon Sawan measures approximately 600 cubic meters per second, but can exceed 4,000 cubic meters per second during flood peaks. The Chao Phraya delta covers approximately 13,000 square kilometers, with distributary channels including the Tha Chin River branching west and the Bang Pakong River system to the east. Four major dams regulate flow on the Chao Phraya system: Bhumibol Dam on the Ping River completed in 1964 with storage capacity of 13,462 million cubic meters, Sirikit Dam on the Nan River completed in 1972 with capacity of 9,510 million cubic meters, and smaller dams on the Wang and Yom rivers. These dams provide irrigation water, generate hydroelectric power, and reduce downstream flooding, though they have altered the river's natural flow regime.
The Mekong River forms Thailand's border with Laos for approximately 976 kilometers along the northern and northeastern regions. The river enters Thailand's border zone in Chiang Rai Province after flowing from its source on the Tibetan Plateau through China and Myanmar. At Chiang Saen in Chiang Rai Province, the Mekong's elevation is approximately 360 meters above sea level. The river drops to roughly 130 meters elevation where it meets the Mun River in Ubon Ratchathani Province before exiting Thailand's border zone and continuing through Cambodia and Vietnam to the South China Sea. The Mekong's width along the Thai border varies from approximately 200 meters in narrow sections to over 1,000 meters where it widens. During the dry season from December through April, discharge at Chiang Saen averages approximately 2,000 cubic meters per second, while monsoon floods from July through October can exceed 20,000 cubic meters per second. The Mekong's primary tributary within Thailand's border is the Mun River, which drains the southern Khorat Plateau and contributes an average annual flow of approximately 6.3 billion cubic meters. The Khong-Chi-Mun river basin covers approximately 117,000 square kilometers in Thailand. The Mekong River Commission, established in 1995 with Thailand, Laos, Cambodia, and Vietnam as members, coordinates water resource management, though upstream dam construction in China and Laos has altered flow patterns and sediment loads.
Phang Nga Bay covers approximately 400 square kilometers of the Andaman Sea between Phuket Island and the mainland in Phang Nga Province. The bay contains approximately 100 limestone karst tower formations rising vertically from the water, some reaching heights of 300 meters. These formations began as coral reefs during the Permian period roughly 280 million years ago, then underwent uplift and erosion. Sea level rise approximately 6,000-8,000 years ago flooded the valley floors, creating the current seascape of towers emerging from shallow water. Tidal range in the bay reaches up to 3 meters, exposing extensive mudflats during low tide. Mangrove forests cover approximately 100 square kilometers along the bay's shorelines, representing one of Thailand's most significant mangrove ecosystems. At least 82 fish species and 18 reptile species inhabit the bay's waters and mangroves. Limestone caves penetrate many of the karst formations, some accessible only by sea kayak during low tide. Khao Phing Kan, known internationally as James Bond Island after appearing in the 1974 film "The Man with the Golden Gun," sits near the bay's center. The adjacent limestone pillar called Ko Tapu rises approximately 20 meters from the water. The bay achieved protected status as Ao Phang Nga National Park in 1981, covering 400 square kilometers. Commercial fishing and tourism both operate within the bay, creating ongoing management challenges.
The Isthmus of Kra represents the narrowest section of the Malay Peninsula, measuring approximately 44 kilometers between the Gulf of Thailand on the east and the Andaman Sea on the west at its narrowest point in Chumphon Province. The isthmus extends for roughly 400 kilometers from approximately 10 degrees north latitude near the Myanmar border southward to where the peninsula widens again in southern Thailand. The Tenasserim Hills run along the western side of the isthmus, with peaks generally between 500 and 1,000 meters in this section. The eastern lowlands slope gradually to the Gulf of Thailand. Various proposals for cutting a canal across the isthmus date back at least to the 17th century during the Ayutthaya Kingdom, with renewed interest at multiple points including a 1973 study commissioned by the Thai government. Engineering assessments estimated such a canal would need to be approximately 50-100 kilometers long depending on the route, accommodate ships up to 150,000 deadweight tons, and cost tens of billions of dollars. No canal has been constructed, and maritime traffic continues using the Strait of Malacca south of the peninsula. The isthmus functions biogeographically as a barrier to some species and a corridor for others, with distinct fauna on either side of the narrowest section.
The Gulf of Thailand covers approximately 320,000 square kilometers with an average depth of 45 meters and maximum depth of 80 meters, making it a relatively shallow marginal sea of the South China Sea. The gulf extends roughly 800 kilometers from north to south and 560 kilometers east to west. Thailand's coastline on the gulf runs approximately 1,660 kilometers from the Cambodian border near Trat Province, north along the eastern coast, around the head of the gulf near Bangkok, and southwest down the western coast to Chumphon Province. Major rivers entering the gulf include the Chao Phraya, Bang Pakong, Tha Chin, and Mae Klong from Thailand. Surface water temperature averages 28-30 degrees Celsius year-round. Salinity measures approximately 30-32 practical salinity units near the surface, lower than oceanic levels due to freshwater input from rivers. Tidal range at the gulf's head near Bangkok reaches approximately 3 meters during spring tides. The gulf's shallow depth and river inputs create productive fishing grounds, with Thailand's catch from the gulf historically reaching over 2 million metric tons annually, though catches have declined since the 1990s due to overfishing. The gulf floor consists primarily of sand and mud sediments, with limited coral reef development compared to the Andaman Sea. Underwater archaeological sites in the gulf include shipwrecks and submerged settlements from periods when sea level was lower.