1591 (MDXCI) was a common yearstarting on Tuesday of the Gregorian calendarand a common year starting on Friday of the Julian calendar, the 1591st year of the Common Era (CE) and Anno Domini (AD) designations, the 591st year of the 2ndmillennium, the 91st year of the 16thcentury, and the 2nd year of the 1590s decade. As of the start of 1591, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.

• August 30 – The Battle of Flores begins
• November 11 – King Henry IV of France begins the siege of Rouen.