Its really neither. It can be used to block a specific kind of pain called
"wind-up" pain. And at extreme doses can be used as an anaesthetic. I mean,
if you HAD to choose from either of those you'd call it a tranquilizer. But
really its a dissociative anaesthetic.

*Is Ketamine a tranq or painkiller in humans?

It's a dissociative anesthetic. Why do you want to know?