'Sons of Anarchy' Star Johnny Lewis -- Birth Certificate Reveals ... HE'S THE DADDY

TMZ has obtained an EXPLOSIVE document in the mystery behind Johnny Lewis' alleged love child -- the birth certificate which shows the "Sons of Anarchy" star was listed as the kid's dad.

We broke the story ... Johnny -- who died following a murderous rampage back in 2012 -- believed he fathered a baby girl in 2010 with Diane Gaeta ... an actress who once appeared on "Sons."

But after he died, Johnny's father filed court documents DISPUTING Johnny's belief -- claiming Johnny had NO offspring and therefore his entire estate should be divided amongst his parents.

Now, TMZ has unearthed the birth certificate, which was filed in California -- and it lists Johnny as the father. The document was also signed by Diane.

The document could play a HUGE role in the division of Johnny's estate -- because, if the judge finds that Johnny is, in fact, the little girl's father ... his entire estate would go to her.

Stay tuned ...