Carrie Underwood gave New Kids On The Block fans an unexpected treat on Wednesday night (17May17) when the country singer joined the group onstage in her adopted Nashville, Tennessee.

A longtime New Kids fan, the Jesus Take the Wheel singer jumped at the chance to step onstage with the boy band when the Total Package Tour came to town.

And she was only too happy to help the bandmates surprise Jordan Knight on his birthday, singing Happy Birthday to the star at the Bridgestone Arena, to celebrate him turning 47.

"We decided to bring Carrie Underwood out to sing Happy Birthday to Jordan," Donnie Wahlberg told the crowd, as the country star stepped onstage and urged fans to sing along "like you mean it".

Donnie then stunned the group's special guest, stating, "Since Carrie's one of our biggest fans for her entire lifetime and drives around the country to see us, which we still can't believe happened, now we're going to let Jon (Knight, bandmate) sing Happy Birthday to you, even though it's not your birthday."

Carrie wasn't the only surprise act to join New Kids onstage on Wednesday - fellow country musicians Big & Rich were invited to perform their track Save a Horse (Ride a Cowboy).

Meanwhile, Underwood also spent time hanging out backstage with Paula Abdul, who is opening for NKOTB on tour.

She posted a shot of herself with Paula on Instagram and captured the image: "Well, look who I ran into backstage at #TheTotalPackage Tour! So cool to watch her do her thing!"

Carrie became friends with Abdul after winning American Idol, the TV talent show on which the pop veteran was a judge.