Jim's sister-in-law, Flo Seals, tells TMZ ... he died Monday evening at his home outside Nashville. She says Jim had several health issues, primarily with his kidneys and lungs, and had just gotten home from the hospital.

Flo says there was a nurse at the house, because they were going to begin hospice care for Jim. He was only home for a few hours before he passed away -- but his family was by his side.

Seals and Crofts are best known for their soft/yacht rock smash "Summer Breeze," but Jim and his musical partner Darrell "Dash" Crofts reeled off a huge string of hits in the 70s.

They also charted with classics like "Diamond Girl" and "Get Closer." They first split up in 1980, but reunited several times in later years, primarily to do live shows as yacht rock became a phenomenon.

They recorded their final album, "Traces," in 2004.

Jimmy's brother, Eddie Seals, tells us ... he was a good person and he was very close to his family. Eddie says, his music was the most important thing he gave to the world and that will be remembered forever.

Jim Seals was 80.