Prof. Harold Hill: A dynamic con man and traveling salesman, Prof. Hill comes to River City, Iowa, and creates a hysteria about the "trouble" of the new pool table in town in order to sell band instruments and uniforms. He promises to form a wholesome boy's band, though he can't read a note of music. His main ambition, besides fleecing the good citizens of their cash, becomes trying to win over the local librarian, with whom he eventually falls in love.
Marian Paroo: A sometimes stuffy librarian and piano teacher, Marian sees through Harold's charade and sets out to expose him as a fraud, but inevitably falls for him.
Winthrop: Marian's lisping younger brother, Winthrop's excitement about the boy's band is part of the reason behind Marian's transformation.
Mrs. Paroo: Marian and Winthrop's charming Irish mother.
Mayor George Shinn: A blustery politician and mayor of River City.
Eulalie Mackecknie Shinn: Mayor Shinn's peacock wife, she is easily won over by Prof. Hill when he appoints her the head of the "Ladies' Auxiliary Society for the Classic Dance."
The Barbershop Quartet: Four bickering school board members (Olin Britt, Oliver Hix, Ewart Dunlop, and Jacey Squires).
Pick-a-Little Ladies: Eulalie's gossipy friends. The Pick-a-Little Ladies include Alma Hix, Mrs. Squires, Maud Dunlop, and Ethel Toffelmier (the "pianola" girl).
Marcellus: Harold's old friend and former partner, Marcellus is now a resident of River City. He has given up his con man ways and is dating his boss's niece, Ethel Toffelmier.
Amaryllis: Marian's young piano student.
Tommy Djilas: The town troublemaker, Tommy is secretly dating the mayor's oldest daughter, Zaneeta. Harold makes him assistant bandleader.
Zaneeta: The mayor's oldest and slightly daffy daughter.
Charlie Cowell: A traveling anvil salesman, Cowell tries to expose Harold as an imposter.