The town covers completely the area of the tambon Khao Niwet (เขานิเวศน์).

Ranong is on the estuary of the Pak Chan (or Kraburi) River, opposite Myanmar's Kawthaung (formerly Victoria Point).

The Tenasserim Hills rise directly to the east of Ranong, and another small ridge runs along the edge of the estuary to the town's north.

There is little variation in the temperature throughout the year, although the pre-monsoon months (February to April) are somewhat hotter in the day.

However, Ranong's position to the west of the Tenasserim Hills means that the monsoon season's rains are greatly amplified, resulting in torrential rains from May until October, and significant rainfall in the transition months of April and November.