Ex-NFL Star Rudi Johnson Dead At 45

7:21 AM PT -- Cops tell TMZ Sports ... Rudi Johnson died by suicide in Florida just after midnight on Tuesday.

Former Cincinnati Bengals running back Rudi Johnson has died.

A family source for the ex-NFL star tells TMZ Sports ... he passed away this week.

A cause of death is not yet known ... but we're told he had been recently struggling with mental health issues and the possible effects of CTE that he could not overcome.

The family source says prior to his death, Johnson was doing everything he could to help people on and off the field ... and that's what mattered most to him.

He was 45 years old.

Johnson was picked in the fourth round of the 2001 NFL Draft by the Bengals ... and after playing sparingly in his first two seasons in Cincy, he took off in Year 3.

As a 24-year-old, he rushed for 957 yards and nine TDs ... and in his ensuing three seasons on the gridiron, he ran for over 4,000 yards and 36 scores.

Johnson's NFL career ultimately came to an end following a run with the Detroit Lions in 2008.

Prior to his pro career, he starred at Auburn University -- where he won SEC Player of the Year Honors one time.

Johnson created his foundation after turning pro ... which focused on helping children and families through community programs.

Bengals president Mike Brown addressed Johnson's passing on Tuesday ... saying, "Rudi was a fine person and an excellent running back for us."

"He was dependable and productive as a player, and very popular among his teammates. Everyone liked him and saw him as a dear friend. We are deeply saddened by his passing."

RIP.