Tyrannicide is the killing or assassination of a tyrant or unjust ruler, purportedly for the common good, and usually by one of the tyrant's subjects. Tyrannicide was legally permitted and encouraged in the Classical period. Often, the term tyrant was a justification for political murders by their rivals, but in some exceptional cases students of Platonic philosophy risked their lives against tyrants. The killing of Clearchus of Heraclea by a cohort led by his own court philosopher is considered a sincere tyrannicide. The killers are also called "tyrannicides".

The term originally denoted the action of Harmodius and Aristogeiton, who are often called the Tyrannicides, in killing Hipparchus of Athens in 514 BC. In modern terms, this term, as in taking life of another person, is a criminal act and lawbreaker is subjected to police arrest and trialed by court.