Activities of radon (222Rn) in groundwater were continuously monitored in a saline aquifer from September 2010 to July 2011. The activities of 222Rn ranged from 200 to 4300 Bq m−3, with a large seasonal variation. Because the activity of 222Rn in seawater is low, 222Rn in saline groundwater must be produced in the aquifer from radium (226Ra) in rocks and sediments. The 222Rn activities were higher in the warm-dry seasons (September–November 2010 and April–May 2011) when the saline aquifer was stable. In contrast, the lowest 222Rn activities were observed in the cold-dry season (December 2010 and January–March 2011), because of the effective exchange between groundwater and seawater. In addition, sudden decreases of 222Rn activities coincided with episodic drops in groundwater temperatures. These results reveal that lower seawater temperature in winter may result in density-driven seawater intrusion. During the wet season (June–July 2011), 222Rn activities were more clearly affected by semi-monthly and diurnal tidal pumping, showing higher 222Rn activities during low and spring tides. Such a tidal effect was not clearly observed during the warm-dry and cold-dry seasons. This result implies that the residence time of SGD in coastal zones is significantly affected by seasonal changes in driving forces such as tidal pumping and seawater intrusion. Copyright © 2016 John Wiley & Sons, Ltd.