"I'm in Love with a Wonderful Guy" is a show tune from the 1949 Rodgers and Hammerstein musical South Pacific.
It was first introduced by Mary Martin in the original Broadway production and sung by Mitzi Gaynor in the 1958 film adaptation.
In the show, Nellie sings this song after being convinced that, despite the fact that Emile killed a man accidentally in a fight, there is good in him—and so she is elated to be in love with him.
Rodgers and Hammerstein were as inspired by Mary Martin's personality as they were by the character of Nellie Forbush to write this song.
Hammerstein wrote this song to express the exuberance of Nellie's character at this point in the show, which carried over to Martin's performance.