Billboard: Music's Ten Most Powerful Couples
Country singers Faith HIll and her husband Tim McGrawREUTERS/Mario Anzuoni

Faith Hill turned 50 on Thursday. And to celebrate her big day, Tim McGraw penned an adorable note to his wife of 21 years.

"Happy birthday to the most beautiful girl in the world," McGraw wrote alongside a photo of Hill on Instagram. "You are the most remarkable person that has ever been in my life."

"Our daughters could not have a better role model. I am truly in love with you more everyday," he said and added, 'Can't wait to see what 'the rest of our life' has in store for us."

Hills' big day comes after McGraw celebrated his 50th birthday earlier this May. Hill shared an adorable black and white photo that shows the couple hugging each other, with McGraw kissing his wife on her forehead while half submerged in water.

"Never better baby!!!!," Hill wrote alongside the snap she shared on the photo-and-video sharing application. "Happy 50th Birthday my love. This world is a better place because of you. I know you are having one of the best days of your life!!!!!"

Hill and McGraw have been together for over 20 years. They share three daughters — Audrey, 15, Maggie, 19 and Gracie, 20.

The couple revealed the secret to their happy marriage earlier this year.

"Just say 'Yes, ma'am' a lot,'" McGraw told Architectural Digest.

"Not saying 'yes' all the time!" joked Hill.

Last year, while talking about their tour with Extra, McGraw said, "I'm excited to be out there and see her sing — that's the biggest part for me."