1900 (MCM) was an exceptional common year starting on Monday of the Gregorian calendar and a leap year starting on Saturday of the Julian calendar, the 1900th year of the Common Era (CE) and Anno Domini (AD) designations, the 900th year of the 2nd millennium, the 100th and last year of the 19th century, and the 1st year of the 1900s decade. As of the start of 1900, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.

As of March 1 (O.S. February 17), when the Julian calendar acknowledged a leap day and the Gregorian calendar did not, the Julian calendar fell one day further behind, bringing the difference to 13 days until February 28 (O.S. February 15), 2100. The year 1900 also marked the Year of the Rat on the Chinese calendar.