Darlene Bieber said the 80-year-old Grand Ole Opry star, known for his trademark rhinestone stage outfits, "is asking for prayers from his friends and fans."
She had no other information, Bieber said Thursday.
WSMV-TV reported that Wagoner was admitted to the hospital earlier this week for observation.
He was hospitalized for two weeks in July 2006 after suffering a stomach aneurism.
In May, he celebrated his 50th year on the Grand Ole Opry, the long-running live country music show. He helped launch Dolly Parton's career by hiring her as his duet partner in 1967.
Wagoner's career took an upturn this year when he signed with ANTI-records, an eclectic Los Angeles label best known for alt-rock acts such as Tom Waits, Nick Cave and Neko Case. He released the album "Wagonmaster," earning Wagoner some of the best reviews of his career.
Wagoner was the opening act for the White Stripes at a sold-out show at Madison Square Garden in Manhattan this summer.