Radon-222 has been measured in groundwater, dwellings, and atmosphere of an inhabited area adjacent to the granitic Aja heights of Hail province, Saudi Arabia. The measurements were carried out in the field using a RAD7 instrument. Twenty-eight water samples, collected from drilled wells scattered in the region, were analyzed. Radon-222 concentration ranged from 2.5–95 kBq m−3 with an average value of about 30.3 kBq m−3. The higher values were found in wells drawing water from granitic aquifers. Indoor 222Rn was measured in 20 dwellings of rural areas in Hail city and other towns. Concentrations ranged from 12–125.6 Bq m−3, with an average value of 54.6 Bq m−3. Outdoor air 222Rn was measured at 16 sites, with values ranging from 6.2–13.3 Bq m−3, with an average value of 10.5 Bq m−3. The estimated average effective dose due to inhalation of 222Rn released from water was 0.08 mSv y−1. The estimated average annual effective dose due to indoor 222Rn was 1.35 mSv, which lies below the effective dose range (3–10 mSv) given as the recommended action level. Based on the average dose rate values, the excess lifetime cancer risk values were estimated as 69.8 × 10−4 due to indoor radon and 13.4 × 10−4 due to outdoor radon.