Deacon Phillippe teased the track on Instagram Monday with a promotional black and white photo of himself staring off into the distance.
"Long Run" also features Scottish singer-songwriter Nina Nesbitt.
"It's SOOOOO good!!" Witherspoon commented on the post.
Witherspoon shares Deacon Phillippe and her 20-year-old daughter, Ava Phillippe, with ex-husband, Ryan Phillippe. She is also the mother of 7-year-old Tennessee Toth with her husband Jim Toth.