Carrie Underwood is accused of being a "Sunday Night Football" theme song-stealer -- another singer claims Carrie straight jacked her hit, "Game On" ... and used it without permission.
The woman behind the suit is Heidi Merrill -- a singer who had success with the sports-themed song "Cornhusker Strong" back in 2015.
Merrill says she wanted to ride the wave and make more sports-themed music so she came up with the song "Game On" in 2016 and posted it to YouTube in March 2017.
In her suit, Merrill says she met Carrie Underwood's producer, Mark Bright, at an event in Nashville in August 2017 and asked if Carrie would be looking to create a new song for the 2018 NFL "Sunday Night Football" show open.