The island of Sri Lanka lies in the Indian Ocean, southwest of the Bay of Bengal. It is separated from the Indian subcontinent by the Gulf of Mannar and the Palk Strait.

According to Hindu mythology, a land bridge to the Indian mainland, known as Rama's Bridge, was constructed during the time of Rama. Often referred to as Adam's Bridge, it now amounts to only a chain of limestone shoals remaining above sea level. According to temple records, this natural causeway was formerly complete, but was breached by a violent storm (probably a cyclone) in 1480.

The pear-shaped island consists mostly of flat-to-rolling coastal plains, with mountains rising only in the south-central part. Amongst these are Sri Pada and the highest point Pidurutalagala (also known as Mt Pedro), at 2,524 meters (8,281 ft). The Mahaweli ganga (Mahaweli river) and other major rivers provide fresh water.

At its maximum, Sri Lanka is 435KM / 235 miles long (North to South) and 225KM / 140 miles wide (East to West).

The climate is tropical, characterized by monsoons: the northeast monsoon lasts from December to March, the southwest June to October. The lowest gravitational field on Earth lies just off the coast of Sri Lanka

Sri Lanka is one of the world's bio-diversity hot-spots. Its forests are among the most floristically rich in Asia and for some faunal groups, it has the world's highest density of species diversity. The southwest, where the influence of the moisture-bearing southwest monsoon is strongest, is home to the Sri Lanka lowland rain forests. At higher elevations they transfer to the Sri Lanka mountain rain forests. Both these tropical moist forest ecoregions are very similar to those of India's Western Ghats.

The north and east are considerably drier, lying in the rain shadow of the central highlands. The Sri Lanka dry-zone dry evergreen forests are a tropical dry broadleaf forest ecoregion, which, like the neighbouring East Deccan dry evergreen forests of India's Coromandel Coast, is characterized by evergreen trees, rather than the dry-season deciduous trees that predominate in most other tropical dry broadleaf forests.

These forests have been largely cleared for agriculture, timber or grazing, and many of the dry evergreen forests have been degraded to thorn scrub, savannah, or thickets. Several preserves have been established to protect some of Sri Lanka's remaining natural areas. The island has three biosphere reserves, Hurulu (established 1977), Sinharaja (established 1978), and Kanneliya-Dediyagala-Nakiyadeniya (KDN) (established 2004).