A martyr (Greek: μάρτυς, mártys, "witness", or μαρτυρία, marturia, stem μαρτυρ-, martyr-) is someone who suffers persecution and death for advocating, renouncing, or refusing to renounce or advocate, a religious belief or other cause as demanded by an external party.

In the martyrdom narrative of the remembering community, this refusal to comply with the presented demands results in the punishment or execution of an individual by an oppressor. Accordingly, the status of the 'martyr' can be considered a posthumous title as a reward for those who are considered worthy of the concept of martyrdom by the living, regardless of any attempts by the deceased to control how they will be remembered in advance. Insofar, the martyr is a relational figure of a society's boundary work that is produced by collective memory. Originally applied only to those who suffered for their religious beliefs, the term has come to be used in connection with people killed for a political cause.

Most martyrs are considered holy or are respected by their followers, becoming symbols of exceptional leadership and heroism in the face of difficult circumstances. Martyrs play significant roles in religions. Similarly, martyrs have had notable effects in secular life, including such figures as Socrates, among other political and cultural examples.