Kim Kardashian has a famous ass, and Sarah Paulson got really handsy with it Tuesday night -- literally smacking it again and again and again!!!

Check out the crazy clip ... The two stars are posing for paparazzi pics in front of The Whitby Hotel for the NYC premiere of the Ryan Murphy TV drama, "All's Fair" -- but then things take a very interesting turn. Mikhail Egorov

Sarah starts playfully smacking Kim's derriere in front of everyone, which is pretty darn bold. What was Kim's reaction? Well, Kim seemed unfazed by the ass slaps -- and even laughed them off as she took Sarah's hand in hers, while the pair disappeared inside the hotel. Getty

They were also joined by "All's Fair" costars Naomi Watts and Niecy Nash-Betts , as well as Murphy, the Hulu series cr

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