Yes, pun is a Scrabble word!
...and is worth 5 points in Scrabble, and 8 points in Words with Friends
pun
Verb
1. Make a play on words
Noun
1. A humorous play on words
What Scrabble words can I make with the letters in pun?
2 Letter Words
Verb
1. Make a play on words
Noun
1. A humorous play on words
2 Letter Words