One is the number 1, and it may also refer to:

• 0.999..., a mathematical equivalent of 1
• The year 1 A.D.
• One (pronoun), a personal pronoun in the English language
• 1 (number)
• 1 BC, the year before 1 AD
• 1 (New York City Subway service)
• Hydrogen, with an atomic number of 1
• 1 (film), a 2009 Hungarian film by Pater Sparrow
• 1 (Goodbye), txtspk for "Goodbye"
• 1, a character in the film 9
• The year 2001, or any year ending with 01
• The month of January