Hate mail (as electronic, posted, or otherwise) is a form of harassment, usually consisting of invective and potentially intimidating or threatening comments towards the recipient. Hate mail often contains exceptionally abusive, foul or otherwise hurtful language.

The recipient may receive disparaging remarks concerning their ethnicity, sexuality, gender, religion, intelligence, political ideology, sense of ethics, or sense of aesthetics. The text of hate mail often contains profanity, or it may simply contain a negative, misappropriating message.

Senders of hate mail normally send anonymous letters or pose as someone else (either a different or fictitious individual) in order to avoid being identified and tracked down, as the nature of some hate mail would inevitably result in criminal charges if the sender was identified.