Exclusive

Steve Cash -- known to his millions of fans as the guy behind the "Talking Kitty Cat" videos on YouTube -- is dead by suicide ... TMZ has learned.

The social media star died Thursday morning from a self-inflicted gunshot wound. Steve's wife, Celia DeCosta Cash, wrote on Facebook, "I just lost my best friend, my sidekick, my lover, my mentor, my absolute everything. It hurts me to share this. Please understand, give the family time to grieve, but know that Steve is no longer in pain."

Steve was open with his fans about his mental health battles. Back in September, someone replied to one of his posts saying, "You've been so active on social media lately that at first I thought you were hacked."

Steve replied, "Nah, just bipolar. I'm on a manic up right now. When I go back into depression I'll vacate everything."

He started his sketch comedy series (think "Garfield") on YouTube in November 2007. He voiced every episode in which he talks to his cats as if they were humans. The short but popular videos drew tons of fans ... with 2.43 million subscribers.

In nearly two decades, Steve racked up more than 770 MILLION views and counting. His most popular video is the "Bad! Bad! Bad!" episode from 4 years ago ... with 17 million views. His last video was posted on December 30.