Is Sampson a Montague or Capulet in the play "Romeo and Juliet"?

Generally, Sampson is listed with Gregory in the list of characters, and the 2 are often described as such:

"Two servants of the house of Capulet, who, like their master, hate the Montagues. At the outset of the play, they successfully provoke some Montague men into a fight."

So, your question is a good one, and comes with a simple and straightforward answer: Sampson is a Capulet. Perhaps not by blood, but bound by his word and his work. And, remember, Juliet is our Capulet, while Romeo is our Montague.