Florissant is located at 38°47'20" North, 90°19'21" West (38.788889, -90.322500).
According to the United States Census Bureau, the city has a total area of 11.69 mi2 (30.29 km2). 11.37 mi2 (29.44 km2) of it is land and 0.33 mi2 (0.85 km2) of it is covered by water. The total area is 2.80% water.
Florissant is located in the Central Standard Time (CST) timezone, GMT-06:00. It observes Daylight Saving Time (DST).