Marva Whitney, known as "Soul Sister No. 1," died at age 68.
The singer received that nickname from James Brown when she was a vocalist in the James Brown Revue during the 1960s.
According to an announcement on a Facebook page dedicated to her, Whitney died on Dec. 22 after suffering from complications caused by pneumonia.
"She left us with a legacy that will shine forever," the message included.
Born Marva Ann Manning, Whitney started her career in music with her family's gospel group, The Manning Gospel Singers, according to WTVR.
Whitney reluctantly joined the James Brown Revue in 1967 and explained her involvement with church.
"There was nothing here in Kansas City, so I had to make a decision at that age," said the singer during an interview from 2006 with We Funk Radio.
"I knew this wasn't what I wanted, because I was still playing for the church," Whitney added. "But I made the decision and went to Cincinatti and signed with King Records."
Some of Whitney's hit songs include 1969's Hot 100 "It's My Thing," "Unwind Yourself," and "I Am What I Am."
"It's My Thing" was a response to the Isley Brothers' hit "It's Your Thing" and became her biggest solo hit.
On Twitter, many users are mourning the death of Whitney on Tuesday.
"Marva Whitney, your sound will never be duplicated, may your soul rest in peace," wrote Jermaine.
"R.I.P. James Brown's 'Soul Sister Number 1' Marva Whitney," added Bossip.
Shane posted, "R.I.P Marva Whitney… Real #music heads know… sad loss…"
Meanwhile, James Brown is considered one of the founding fathers of funk music. His 6-decade career saw hits songs such as "The Boss," "I Got You (I Feel Good)," and "Get Up Offa That Thing."
The singer and songwriter died six years earlier than Whitney on Dec. 25, 2006.