RJ Mitte Cause of Death

1:42pm PT -- RJ's reps tell TMZ ... "It is with profound sadness we share the tragic passing of beloved actor, philanthropist, and passionate disability rights advocate RJ Mitte."

"RJ Mitte was best known for his role as Walter 'Flynn' White Jr. on AMC’s critically acclaimed, award-winning series Breaking Bad. Drawing from his own experience living with cerebral palsy, Mitte brought authenticity to the role and built a platform on disability awareness and inclusion," his reps continue, "Beyond entertaining friends and family around the world, he served as an ambassador for United Cerebral Palsy and Shriners Children’s."

RJ Mitte -- who played Walter White Jr. on "Breaking Bad" -- is dead ... TMZ has learned.

A county official confirmed to TMZ that RJ died Monday in Texas. We're told he died by suicide.

Law enforcement tells us the death investigation is ongoing.