Thailand occupies 513,120 square kilometers in mainland Southeast Asia, stretching 1,648 kilometers from its northern border with Myanmar and Laos to its southern tip at the Malaysian frontier. The country touches four nations: Myanmar along a 2,416-kilometer western and northern border, Laos across 1,845 kilometers to the north and east, Cambodia along 817 kilometers to the southeast, and Malaysia across 595 kilometers in the far south. Two bodies of water define Thailand's maritime geography: the Gulf of Thailand curves along 1,840 kilometers of the eastern and southern coastline, while the Andaman Sea meets 865 kilometers of the southwestern shore. The nation spans latitudes from approximately 5.6 degrees north at its southernmost point to 20.5 degrees north at the Myanmar border, placing the entire country within the tropics. Longitudinally, Thailand extends from 97.3 degrees east to 105.6 degrees east, creating a maximum width of approximately 780 kilometers in the north.
The physiography divides into five distinct regions, each shaped by different geological processes. The northern highlands consist of parallel mountain ranges running north-south, formed by tectonic collision between the Indian and Eurasian plates that began approximately 50 million years ago. These ranges include the Daen Lao Range along the Laos border, the Thanon Thongchai Range forming much of the Myanmar boundary, and the Phi Pan Nam Range running through the interior. Doi Inthanon, the country's highest point at 2,565 meters above sea level, rises in Chom Thong District of Chiang Mai Province within the Thanon Thongchai system. The mountain received its current name from King Inthawichayanon of Chiang Mai, the last ruler of the semi-independent northern kingdom before full incorporation into Siam in 1899. Average elevations in the northern highlands range from 500 to 1,200 meters, with numerous peaks exceeding 2,000 meters. The geology consists primarily of granite intrusions, metamorphic rocks including schist and gneiss, and limestone formations that produce the region's characteristic karst topography.
The Central Plains occupy approximately 100,000 square kilometers between the northern mountains and the Gulf of Thailand, representing Thailand's agricultural heartland. The Chao Phraya River, formed by the confluence of the Ping, Wang, Nan, and Yom rivers at Nakhon Sawan, flows 372 kilometers from this junction point to the Gulf of Thailand near Bangkok. The river drains a watershed of approximately 160,000 square kilometers and discharges an average of 30 cubic kilometers of water annually into the gulf. Sediment deposition from the Chao Phraya and its tributaries over the past 6,000 years built the delta plain, which advances into the gulf at approximately 3 meters per year. Elevation across most of the Central Plains ranges from 2 to 15 meters above sea level, creating minimal topographic variation. The Mae Klong and Tha Chin rivers parallel the Chao Phraya to the west, draining additional areas of the central lowlands. Soil composition in the plains consists of recent alluvium, primarily clay and silt particles that retain moisture and nutrients effectively. The Bangkok Plain specifically occupies the lowest portion of the Central Plains, where elevation drops below 2 meters over extensive areas, requiring substantial infrastructure to manage seasonal flooding.
The Khorat Plateau comprises approximately one-third of Thailand's total area, covering roughly 170,000 square kilometers in the northeast. The plateau tilts gently from south to north, with elevations ranging from 200 meters along the Mekong River boundary to approximately 300 meters at the southern edge where the Phetchabun and Dong Phaya Yen mountain ranges form the barrier to the Central Plains. The Phu Phan Range runs east-west through the plateau's interior, creating additional topographic variation with peaks reaching 600 to 700 meters. The Mekong River defines Thailand's northeastern and eastern boundary with Laos for approximately 976 kilometers, flowing at elevations between 90 and 200 meters above sea level through this section. The Chi and Mun rivers drain the plateau interior, both flowing eastward to join the Mekong in a dendritic pattern that reflects the underlying sedimentary geology. Bedrock consists of Mesozoic sandstone and mudstone formations from the Khorat Group, deposited between 200 and 130 million years ago in a continental basin. These sedimentary rocks weather into sandy, acidic soils with low water retention, making agriculture more challenging than in the Central Plains. Salt deposits occur in numerous locations beneath the plateau surface, a legacy of ancient marine transgressions, and groundwater in many areas exceeds 1,000 milligrams per liter of total dissolved solids.
The eastern region occupies approximately 32,000 square kilometers between the Khorat Plateau and the Gulf of Thailand, characterized by a series of coastal plains and low mountain ranges. The Cardamom and Chanthaburi mountains extend from Cambodia into Thailand's eastern provinces, with peaks in the San Kamphaeng Range reaching 1,556 meters at Khao Soi Dao. These mountains receive the highest rainfall totals in Thailand, with some stations recording over 4,500 millimeters annually. Rivers including the Bang Pakong drain westward into the upper gulf, while numerous shorter streams flow directly into the sea along the eastern coast. The coastline from the mouth of the Chao Phraya to the Cambodian border extends approximately 400 kilometers, characterized by a mix of mudflats, mangrove forests, and sandy beaches depending on local wave energy and sediment supply. Islands in the eastern gulf include Koh Samet, Koh Chang, and Koh Kut, representing exposed portions of the continental shelf that was emergent during lower sea levels of the Pleistocene ice ages. Basement geology consists of Precambrian and Paleozoic metamorphic and igneous rocks overlain by younger sediments in the coastal lowlands.
The southern region, sometimes called the Peninsula, extends approximately 1,200 kilometers from the Isthmus of Kra to the Malaysian border, tapering from widths exceeding 200 kilometers in the north to less than 10 kilometers at certain points. The Isthmus of Kra, located at approximately 10 degrees north latitude, represents the narrowest point of the entire Malay Peninsula at roughly 44 kilometers between the Gulf of Thailand and the Andaman Sea. The Phuket Range runs along the peninsula's western side, parallel to the Andaman coast, while the Nakhon Si Thammarat Range extends along the eastern portion. Elevations in these ranges typically reach 500 to 1,000 meters, with peaks in the Nakhon Si Thammarat Range exceeding 1,500 meters. The Kra Buri River valley marks the location of the isthmus, where engineers have periodically proposed canal construction to connect the two seas, though no such project has proceeded beyond planning stages. Geology of the peninsula includes granite intrusions from the Cretaceous period, older metamorphic rocks, and extensive limestone formations that create karst landscapes both inland and along the coasts.
Phuket Island covers 576 square kilometers off the peninsula's Andaman coast, representing Thailand's largest island. The island measures approximately 48 kilometers north to south and 21 kilometers at its widest east-west extent. A causageway connecting Phuket to the mainland was constructed in the 1960s, replacing earlier ferry service. The island's interior consists of low granite hills reaching 529 meters at Mai Thao Sip Song, while the coastline features a mix of rocky headlands and sandy bays. Phang Nga Bay, located between Phuket and the mainland to the northeast, contains approximately 100 limestone karst islands rising vertically from water depths of 20 to 40 meters. These formations resulted from dissolution of the limestone bedrock during periods of lower sea level, followed by flooding when ocean levels rose after the last ice age ended approximately 11,000 years ago. Khao Phing Kan, popularly known as James Bond Island after appearing in "The Man with the Golden Gun" in 1974, exemplifies the bay's karst morphology with two limestone masses leaning against each other.
The Similan Islands comprise nine granite islands located 70 kilometers offshore in the Andaman Sea, extending in a north-south line over approximately 25 kilometers. The Thai government designated the islands as Mu Ko Similan National Park in 1982, protecting 140 square kilometers including marine areas. The islands represent exposed portions of the Phuket Range granite batholith, which extends beneath the Andaman Sea. Above-water portions of the islands total approximately 26 square kilometers, with the largest island, Koh Similan, covering roughly 4 square kilometers. The islands lack permanent surface water and supported no indigenous human population prior to modern tourism and park management. Rock formations on the islands include large granite boulders weathered into rounded shapes, particularly on the northwestern portions where monsoon wave action has been most intense. Water depths between the islands and the mainland range from 15 to 45 meters over the continental shelf.
Koh Samui covers approximately 228 square kilometers in the Gulf of Thailand, located 35 kilometers east of the Surat Thani coast. The island measures roughly 25 kilometers at its widest point east to west and 21 kilometers north to south. A mountainous interior rises to 635 meters at Khao Pom, with most development concentrated along coastal plains and the narrow valleys between hills. Koh Phangan lies 15 kilometers north of Koh Samui, covering 125 square kilometers with a similarly mountainous topography reaching 627 meters at Khao Ra. The channel between the two islands maintains depths of 30 to 60 meters. Koh Tao, located 45 kilometers north of Koh Phangan, covers only 21 square kilometers and rises to 379 meters elevation. These three islands form the core of the Samui Archipelago, part of the ancient basement complex that extends beneath the gulf. Granite and metamorphic rocks exposed on the islands date to the Permian and Triassic periods, approximately 250 to 200 million years old. The islands remained sparsely populated until coconut cultivation expanded in the early 20th century, with Koh Samui's first road completed in 1973.
The Phi Phi Islands consist of six islands located 45 kilometers southeast of Phuket and 40 kilometers west of Krabi. Phi Phi Don, the larger island at 9.73 square kilometers, features two mountainous sections connected by a narrow sandy isthmus approximately 800 meters wide. The island's peaks reach 497 meters, consisting of limestone that forms part of the same karst system as Phang Nga Bay. Phi Phi Leh, covering 2 square kilometers immediately south of Phi Phi Don, rises to 180 meters and contains Maya Bay on its western side, a 250-meter-wide beach enclosed by 100-meter limestone cliffs. Maya Bay closed to visitors in June 2018 to allow ecosystem recovery after tourism impacts degraded coral and beach conditions, reopening with daily visitor limits in January 2022. The four smaller islands in the group—Koh Bida Nok, Koh Bida Nai, Koh Yung, and Koh Pai—total less than 1 square kilometer combined and consist entirely of steep limestone cliffs with minimal vegetation.
Railay Beach occupies a small peninsula in Krabi Province accessible only by boat due to limestone cliffs blocking land approaches. The peninsula measures approximately 1.5 kilometers east to west and 500 meters north to south, with four distinct beach areas facing different directions. The limestone headlands rise 200 to 300 meters vertically from the sea, part of the Khao Phanom Bencha Range that extends along this section of coast. The cliffs contain over 700 established rock climbing routes ranging from beginner to advanced grades, making Railay one of Thailand's primary climbing destinations since route development began in the late 1980s. The limestone formed during the Permian period approximately 280 million years ago when the region was covered by a shallow tropical sea.
Thailand's river systems drain an area of approximately 520,000 square kilometers, including portions of neighboring countries. The Mekong River forms Thailand's border with Laos and Myanmar for sections of its course but drains only about 23 percent of Thailand's territory directly through its tributaries. The Chao Phraya basin drains approximately 35 percent of the country, representing Thailand's largest river system entirely within national boundaries. The river carries an average sediment load of 11 million tons per year to the Gulf of Thailand, though construction of Bhumibol Dam on the Ping River in 1964 and Sirikit Dam on the Nan River in 1972 reduced sediment transport by approximately 40 percent. These dams, with storage capacities of 13.5 and 9.5 billion cubic meters respectively, regulate flow for irrigation, flood control, and hydroelectric generation. Average discharge at the Chao Phraya's mouth varies from 400 cubic meters per second during the dry season to over 4,000 cubic meters per second during peak monsoon flow, though the dams have dampened these extremes. The river's gradient drops approximately 30 centimeters per kilometer through the lower plain, creating a slow-moving river that historically meandered extensively before channelization and levee construction.
The Mekong River, measuring 4,350 kilometers from source to sea, forms 976 kilometers of Thailand's northeastern and eastern boundary. The river drains 795,000 square kilometers across six countries, with Thailand's northeastern region contributing approximately 184,000 square kilometers to the basin. Average discharge where the river borders Thailand ranges from 1,700 cubic meters per second during the dry season to over 20,000 cubic meters per second at peak monsoon flow, though measurements vary by location along the border. The river drops approximately 8 centimeters per kilometer through this section, creating sufficient energy to transport sand and gravel during high flows. Sediment loads in the Mekong average 160 million tons annually at the delta, though dam construction in China and Laos has reduced this figure since the 1990s. The river's annual flood pulse historically defined agricultural patterns across the Khorat Plateau, with recession agriculture practiced along the riverbanks as water levels dropped after the monsoon. Sand bars and islands in the river channel shift position during flood events, and the main channel has migrated laterally by up to several hundred meters at some locations over the past century.
Khao Yai National Park, established in 1962 as Thailand's first national park, protects 2,168 square kilometers of forest in the Dong Phaya Yen Mountains. The park's elevation ranges from 400 meters in the valleys to 1,351 meters at Khao Rom, creating habitat diversity that supports 67 mammal species and over 300 bird species. The park lies approximately 120 kilometers northeast of Bangkok at the junction of four provinces: Saraburi, Nakhon Nayok, Nakhon Ratchasima, and Prachinburi. UNESCO designated Khao Yai and four associated protected areas as the Dong Phayayen-Khao Yai Forest Complex World Heritage Site in 2005, recognizing the combined 6,155 square kilometers as one of mainland Southeast Asia's largest intact forest blocks. Average annual rainfall in the park exceeds 2,500 millimeters, with the wettest months from May to October. The park's geology consists primarily of sandstone and shale from the Triassic and Jurassic periods, overlain by volcanic rocks from eruptions that occurred 30 to 40 million years ago. Haew Narok Waterfall in the park's south drops 150 meters in three tiers, representing the park's most prominent water feature.
Erawan National Park covers 550 square kilometers in Kanchanaburi Province, established in 1975 primarily to protect the seven-tiered Erawan Waterfall and surrounding forest. The park's elevation ranges from 200 meters along the Mong Lai River to 996 meters in the Tenasserim Range foothills. The Erawan Waterfall drops a total of 200 meters over a linear distance of approximately 1,500 meters, with each tier occupying a distinct elevation separated by stream reaches flowing over bedrock. The waterfall's name refers to Erawan, the three-headed elephant of Hindu mythology, attributed to a perceived resemblance between the top tier's rock formation and an elephant's head. The park receives approximately 1,500 millimeters of rainfall annually, with distinct wet and dry seasons affecting waterfall flow volumes. Limestone caves occur throughout the park, with Phartat Cave and Ta Duang Cave containing stalactite and stalagmite formations. The park's forests transition from mixed deciduous at lower elevations to evergreen forest above 600 meters.