Matt Sayles, AP
The husband-wife team of Keifer and Shawna Thompson learned of their win after they'd performed as part of the ACM Salute to the Military Concert in Las Vegas, where tonight's ceremony takes place, and the pair were visibly moved by the accolade.
"It's been a long, long road," Keifer told the crowd. "This is one of the greatest nights of our lives."
"This is a very bittersweet moment for me," added Shawna. "More sweet than bitter. My dad passed away a month ago. This is for my dad."
Last year, Thompson Square released its self-titled debut album, which hit No. 3 on the country charts and featured the chart-topping hit "Are You Gonna Kiss Me Or Not."
With last night's triumph, Keifer and Shawna have became just the third duo -- after Sugarland and Brooks & Dunn -- to win Vocal Duo of the Year since 1991.
The 47th Annual ACM Awards air tonight at 8PM EST on CBS.