It's an abbreviated form of 'I thank you ', so 'thank' is a verb. But really, it's better to think of 'thank you' as simply a fixed expression in modern English. Clive
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AnonymousWhat part of speech is "thank you"? Is thank a verb in this sentence?For future reference, the term part of speech applies only to one word at a time. The expression thank you comprises two words and therefore two parts of speech: verb and pronoun.