"I Believe in You" is a 1961 song written by Frank Loesser for his musical How to Succeed in Business Without Really Trying, where it was introduced by Robert Morse.
[1] The protagonist of the musical, J. Pierrepont Finch, sings the song to himself in the mirror while shaving.
[2] In the movie version, Rosemary (played by Michele Lee) also sings it to Pierrepont (again played by Morse).