A letter of thanks or thank-you letter is a letter that is used when one person/party wishes to express appreciation to another. Personal thank-you letters are sometimes hand-written in cases in which the addressee is a friend, acquaintance or relative. Thank-you letters are also sometimes referred to as letters of gratitude. These types of thank-you letters are usually written as formal business letters.

Some psychological research indicates that expressing gratitude by writing such letters can have emotional benefits,^[1] but this does not apply to all circumstances.^[2] News media have covered the tradition of handwritten letters of thanks from a cultural perspective, suggesting in particular that the extra effort represented by handwriting (as opposed to text messaging, for example) makes these letters more emotionally significant for sender and recipient alike.^[3][4]

Good thank you letters can benefit from using a recognized format.^[5][6]