Underwood, 27, who was nominated for a Golden Globe this year (for best song) but hasn't acted in movies until now, plays a church youth counselor in the film, which tells the true story of a young surfer in Hawaii who loses her arm in a shark attack.
Underwood's character, Sarah Hill, helps the surfer, Bethany Hamilton (played by AnnaSophia Robb), fight through the tragedy and return to the sport.
"Sarah has been my best friend since I was very young," Hamilton told PEOPLE last year. "After my shark attack, she always knew what to say to push me on."
Soul Surfer is due in theaters April 8.