What is the singular form of 'Monsieur'?

Answer #1 'Monsieur' is already singular. Monsieurs with the "s" is plural. Answer #2 The noun 'Monsieur' combines the possessive adjective 'mon' ['my] and the noun 'sieur' ['lord'] to mean 'mister' or 'sir' nowadays. It's a masculine gender noun that's in the singular. The plural form is Messieurs, as in 'Messieurs et Mesdames' [in reversed order as 'Ladies and Gentlemen' in English].