Mister, usually written in its abbreviated form Mr or Mr. (American English), is a commonly used English honorific for men under the rank of knighthood. The title derived from master, as the equivalent female titles, Mrs., Miss, and Ms, all derived from the archaic mistress. The title master was retained and used for boys and young men, but is now less commonly used. The plural form is Misters, or the abbreviation Messrs (UK) or Messrs. (US) ( /ˈmɛsərz/). This is an English abbreviation of the French "messieurs" (French pronunciation: [mesjø]), sometimes pronounced /ˈmɛsərz/ in English.

When addressing someone directly: Mr. is usually used with the last name only ("May I help you, Mr. Thompson?"). In other circumstances, it can be used with either the last name or the full name ("This is Mr. John Smith."; "Would you please help, Mr Smith?"). In formal written address, it is usually used along with the full name, or with initials and surname.

When a man's name is unknown, "mister" is sometimes used informally by itself in direct address, as in, "Hello, mister," or "Are you all right, mister?" rather than the standard form of address, "Sir". Though not necessarily meant disrespectfully, such use may to some people seem rude.