Undertale took about 2.5 to 3.5 years to make, depending on the source, with many references putting it around 32 months or roughly 3 years and 6 months.

Quick Scoop

Toby Fox began development in 2013 , and the game was released in September 2015 , so the overall timeline lands in that range.

A simple way to say it is: about 3 years of development.