Ex-Notre Dame RB George Atkinson III -- son of former Super Bowl champ George Atkinson II -- died on Monday ... less than 1 year after his twin brother committed suicide.
It's unclear how the 27-year-old passed ... but the former NFL player said losing his brother, Josh, last December took a serious toll on him.
In a letter penned by Atkinson III in October ... he said Josh's death put him "on an emotional roller coaster and in a real dark place."
Atkinson III admitted he attempted to "harm himself" shortly after the death and had to be institutionalized .