The iconic singer passed away Wednesday in Illinois, TMZ has learned. The cause of Rudolph's death is currently unclear, though one person involved tells us it's believed he suffered a heart attack.

Rudolph started singing at a young age in his local church and created The Isley Brothers with his siblings Ronald, O'Kelly, and Vernon when he was just a teenager.

After Vernon was tragically hit by a car while riding his bike at just 13 years old in 1955, the brothers moved to New York to record music ... signing with RCA Records a few years later and debuting with their first song, "Shout" -- arguably one of their most significant tracks.

While the group was still in its infancy, Rudolph married Elaine Jasper in 1958 ... living the married life while the Brothers all went on to garner success with tracks like "Twist & Shout," "This Old Heart of Mine," and "It's Your Thing."

Despite Ronnie often taking the lead in the group, Rudolph got to record lead vocals for some of their songs, too.

As the years went on, The Isley Brothers grew -- adding younger brothers Ernie and Marvin and Rudolph's brother-in-law Chris Jasper into the mix in 1971.

Rudolph left the group in 1989 to carry out his goal of being a Christian minister, but he reunited with the group over the years. They were even inducted into the Rock and Roll Hall of Fame in 1992 ... being given the award by fellow music legend Little Richard.

The group has been cranking out tracks for over half a century -- something many groups can't come close to -- with more recent songs like "Busted" and "Contagious" after Rudolph left.

They also made it into some popular rap hits ... being sampled in Ice Cube's "It Was a Good Day" and Notorious B.I.G.'s "Big Poppa."

Rudolph was 84.