Yes, thank is a Scrabble word!
...and is worth 12 points in Scrabble, and 12 points in Words with Friends
thank
Verb
1. Express gratitude or show appreciation to
What Scrabble words can I make with the letters in thank?
4 Letter Words
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3 Letter Words
2 Letter Words
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Words that can be created with an extra letter added to thank:
S:thanks