Main Menu 01 is the 2nd track in the Undertale Yellow Soundtrack. It plays on the main menu.

The song is a remix of Once Upon a Truthful Time in a similar way to how Start Menu is a remix of Once Upon a Time in Undertale.

Undertale Yellow Soundtrack
1-10 Once Upon a Truthful Time • Main Menu 01 • Enemy Approaching Yellow • Fallen Down Yellow
Ruins Yellow • Howdy! • In Darkness • Main Menu 02 • Justice • Fever Pitch! • Enemy Retreating • Seclusion • Forlorn • House Guest
10-16 Enemy Retreating • Seclusion • Forlorn • House Guest • To Be Continued • Fun-Sized
Bonus Boppin'