Donna Jean Godchaux-MacKay -- the powerhouse vocalist who sang with California psychedelic rock band Grateful Dead -- has died at 78.

The singer died Sunday at Nashville’s Alive Hospital following a long battle with cancer. Her family said in a statement ... "She was a sweet and warmly beautiful spirit, and all those who knew her are united in loss."

"The family requests privacy at this time of grieving. In the words of Dead lyricist Robert Hunter, ‘May the four winds blow her safely home.'"

Donna was best known for her run with the Grateful Dead from 1971 to 1979. Before that, she performed on classics like Elvis Presley’s "Suspicious Minds" and recorded with legends like Cher, Boz Scaggs, and the Dead's own Bob Weir.

After her Grateful Dead years, tragedy struck when her husband and bandmate Keith Godchaux died in an accident ... prompting her to take a break from music. She later found love again with bassist David MacKay.

Donna was 78.

RIP