The former American Idol star was performing the pair's collaboration Remind Me at the Bridgestone Arena when Paisley's voice was suddenly heard singing from backstage.
Underwood looked visibly shocked and stopped mid-song when the star walked out to join her, and she hugged him before continuing with the track.
Paisley subsequently mentioned the surprise in a message to Underwood on Twitter.com, writing, "Howdy. Gotcha," while his friend replied, "Yes you did! Thanks for being there to sing with me tonight! What a great surprise!"