The 1921 Atlantic hurricane season was an active hurricane season, with 12 tropical cyclones forming. Among them, seven became tropical storms, of which five strengthened into hurricanes. Furthermore, two of these strengthened into a major hurricane, Category 3 or higher on the modern-day Saffir–Simpson hurricane wind scale, the most since the 1917 season. The first system, a tropical depression, developed on June 1, while the last, a tropical storm, dissipated on November 25. Of note, three tropical cyclones co-existed with another during the season.

The Atlantic hurricane season is the period in a year, from June 1 through November 30, when tropical or subtropical cyclones are most likely to form in the North Atlantic Ocean. These dates, adopted by convention, encompass the period in each year when most tropical cyclogenesis occurs in the basin. Even so, subtropical or tropical cyclogenesis is possible at any time of the year, and often does occur.