It is bordered by St. Andrew on the west, Portland on the north and the Caribbean Sea to the south.
The land mass of St. Thomas ranges from the peaks of the Blue Mountains and John Crow Mountains down to sea-level. Subsidiary ridges of the Blue Mountain range, running from east to west across the island, come to their eastern end in St. Thomas. These include the Port Royal Mountain Range, which rises in some parts to 1,219.2m (4,000 feet) and stretches from above New Castle, in St. Andrew, to a point near the sea in the Albion area of St. Thomas.
Between the valleys of the Yallahs and Negro River lies the Queensbury Ridge, and to the extreme south of Blue Mountain massif stands the Yallahs Hill, a 730m (2,394 feet) isolated ridge.
The vegetation of western St. Thomas is mostly dry limestone scrub forests at 0 – 381m (0 – 1,250 feet), with the higher rainfall eastern area of the parish showing more luxuriant vegetation. The coastal area of the parish between the Yallahs River and Hector’s River includes large wetland areas such as the Great Morass and the Cow Bay Swamps, as well as significant areas of deciduous forest.
The shoreline is characterised by rocky cliffs and sandy or gravelly beaches and is indented by a number of bays which include Sugar Loaf, Yallahs, Salt Pond, Lyssons, Holland Point, Rocky Point, Canoe and Morant Point.
Behind the coast there are lower mountain rain forests over 381m (1,250 feet) and some elfin woodlands at the highest mountain levels.