No wedding date has been set for the couple, who have been dating for about a year, The Associated Press reports.
"The couple couldn't be happier," said publicist Jessie Schmidt.
Fisher, 29, reportedly proposed at his home Sunday afternoon and then sent messages to his friends with the good news later that day, People magazine reports.
Asked by The Associated Press last month what Underwood's family thought of Fisher, the singer said they were supportive.
"They love me and I would hope my parents would think we raised a good, smart girl, so she's going to do the right thing no matter what it is," she said.
Underwood, 26, has kept her relationship largely under wraps, but she gave Fisher a very public shout-out in the liner notes of her current album, "Play On," which was released Nov. 3.
Exclusive subscription offer
Newsday covers the stories that matter most to Long Islanders. We dig deep to uncover the facts, hold the powerful in check and keep a watchful eye on Long Island.
Your digital subscription, starting at $1, supports local journalism vital to the community.SUBSCRIBE NOW
She wrote, "Thank you [No.] 12 [referring to his jersey number]. You are the most amazing addition to my life! You are such a wonderful person and have had such an amazing hand in the building of this album and in the growth of me as a person. I love you so much! You make my life better in every way!"