'Big Time Rush' Star Kendall Schmidt -- Check Out My Sissy Tattoo

"Big Time Rush" lead singer Kendall Schmidt has taken the phrase "man's best friend" to a whole new level -- by tattooing the face of his dog Sissy on his back ... and TMZ has the photographic proof.

Sources close to Sissy -- Kendall's adorable Boston Terrier -- tell TMZ ... the singer immortalized the pooch's mug onto the left side of his back a few weeks ago at Smith Street Tattoo in Brooklyn.

We're told the ink job took about an hour and was pretty painful -- but it won't be his last ... especially if he ever gets another dog.