Being located in one of the arid regions of Sri Lanka, the climate of Ruhuna National Park is usually hot and dry. The area receives its annual rainfall during the north east monsoon from November to January, and unpredictable inter-monsoonal rains in March/April and September. February is a dry month, with the dry season proper commencing in June and lasting until September and sometimes until mid October.

The mean annual temperature near sea level is 27C, although in the dry season a daily maximum of 37C is not uncommon.

Amongst the rock ridges and monoliths are several natural rock pools that have a charm of their own. Some contain water throughout the year, and have their own development of water plants and fauna.
In the southeast, the Park is bounded by the sea. The many bays carve out an intricate mosaic. Unspoilt natural beaches and sand dunes provide a beautiful environment of undulating and shifting sands. This is surely one of the most spectacular seascapes of Sri Lanka. Far out at sea are two lighthouses, Great and Little Basses, which stand on two submerged ridges by those names and beam a red and white light respectively at night.
Lagoons fringe this part of the coastline, each lined with mangroves and filled with brackish water. The extensive parklands that surround these lagoons offer visitors superb locations for viewing animals and bird life.