Undertale was made by Toby Fox.

Quick Scoop

He is the main creator behind the game and also composed its soundtrack. Some later reporting notes that he developed it mostly on his own, with help from artists such as Temmie Chang.

Extra Detail

  • Toby Fox is credited as the developer most associated with Undertale.
  • The game first released in 2015.
  • Temmie Chang helped with art, according to later coverage of the game’s development.

TL;DR: Toby Fox made Undertale.