Doink the Clown Dead -- Ex-WWE Star Dies at 55

3:15 PM PT TMZ has learned police in Plano, Texas ... where Doink's body was found ... have launched a homicide investigation.

However, sources tell us, the investigation is really just a matter of following S.O.P. -- and that all early indications are his death was accidental.

No weapons were found on the scene. An autopsy is pending.

Ex-WWE star Doink the Clown -- a hugely popular wrestler in the '90s -- was found dead today in Texas.

Doink, real name Matt Osborne, was known for pulling pranks on the other wrestlers ... all while decked out in full greasepaint and a green wig.

Details about Doink's death are unclear at this point -- but his girlfriend reportedly found him dead in her home ... where he'd been staying lately.

After leaving the WWE, he continued wrestling for ECW and independent leagues ... still sporting the clown costume.

More recently, he's been attending wrestling conventions and doing autograph sessions with fans.