Seaside is located at 45°59'35" North, 123°55'15" West (45.993056, -123.921111).
According to the United States Census Bureau, the city has a total area of 4.01 mi2 (10.40 km2). 3.86 mi2 (9.99 km2) of it is land and 0.16 mi2 (0.40 km2) of it is covered by water. The total area is 3.89% water.
Seaside is located in the Pacific Standard Time (PST) timezone, GMT-08:00. It observes Daylight Saving Time (DST).