The plural of monsieur is messieurs.
Monsieur is a French term used to address or refer to a man, typically as a sign of respect or formality. It is equivalent to 'Mr.' in English. Monsieur is commonly used before a person's surname or full name. It is a courteous way of addressing someone in formal or professional settings. The plural form of monsieur is messieurs, which is used when addressing or referring to multiple men in a respectful manner.