The song is called "I Knew It, I Knew You" and will be released in conjunction with the world premiere on Friday.
"I've always dreamed of writing for these characters that I've loved since I was a five-year-old kid watching the first 'Toy Story' movie," Swift wrote.
Both Swift and Pixar have hinted that a collaboration is in the works. The singer's announcement came after a Toy Story-themed countdown, Billboard reported.
"Toy Story 5" will be released in Swedish cinemas on June 17.
