September Affair is a 1950 American romantic drama film directed by William Dieterle and starring Joan Fontaine, Joseph Cotten, and Jessica Tandy.

Marianne "Manina" Stuart (Joan Fontaine), a prominent concert pianist, meets David Lawrence (Joseph Cotten), a businessman, on a flight from Rome to New York.

Thinking their absences will not make any difference to the larger world, they decide to "stay dead" and begin a new life together in Florence.

They make no contact with their families or friends, including Lawrence's wife Catherine (Jessica Tandy) and son David Jr. (Robert Arthur).

She also has contact with piano teacher Maria Salvatini (Françoise Rosay), who agrees not to reveal Manina is very much alive, but continues to tutor her.

Catherine and her son travel to Florence after hearing of this transfer to try to find out any more on David's fate from the woman he gave the money to.

Critic Bosley Crowther, in his review for The New York Times, called this film a "rambling drama" and a "banal adventure" in which producer Hal B. Wallis and his team "have set down a hopelessly silly story in front of some beautiful scenery and a haunting song.