Dr. Arnie Klein -- Michael Jackson's famed dermatologist -- has just taken credit for fathering MJ's son Prince Jackson ... sort of.

Klein posted the cryptic picture on his Facebook profile yesterday, showing Prince side-by-side with an old photo of Klein when he was younger -- presumably to show the resemblance.

Under the photo, Klein wrote the caption, "hmmmmm" -- a wink at people who've been saying for years that Klein provided the sperm that fathered Prince and Paris.

You'll recall, Michael Jackson's ex-wife Debbie Rowe gave birth to Prince in 1997, and daughter Paris the year after. MJ's third child Blanket was born to an anonymous surrogate in 2002.