An unmarked grave is one that lacks a marker, headstone, or nameplate indicating that a body is buried there. It may also include burials that previously had identification but which are no longer identifiable due to weather damage, neglect, disturbance or otherwise. However, in cultures that mark burial sites, the phrase unmarked grave has taken on a metaphorical meaning.

The term has been used to describe former Canadian Indian Residential School cemeteries. "Given the lack of regulations" in the schools' early years, it appears that most Residential School cemeteries "were established informally", resulting in little formal documentation as to their whereabouts. Over time, many cemeteries had been abandoned, disused, and were vulnerable to accidental disturbance and weather damage. As such, the locations of many burial sites, wood grave markers and names of the deceased have been lost.

As a figure of speech, a common meaning of the term "unmarked grave" is consignment to an ignominious end. A grave monument (or headstone) is a sign of respect or fondness, erected with the intention of commemorating and remembering a person.

Conversely, a deliberately unmarked grave may signify disdain and contempt. The underlying intention of some unmarked graves may be to suggest that the person buried is not worthy of commemoration or respect, and should be completely ignored and forgotten, e.g., mass murderers such as Boston Marathon Bomber Tamerlan Tsarnaev.

Unmarked graves have long been used to bury executed criminals as an added degree of disgrace. Similarly, many 18th and 19th century prisons and mental asylums historically used numbered (but otherwise featureless) markers in their inmate cemeteries, which allowed for record-keeping and visitations while also minimizing the shame associated with having one's family name on permanent display in such a disreputable context. Plot E at Oise-Aisne American Cemetery (consisting entirely of soldiers executed for rape and/or murder) is a rare example of this policy persisting into the 20th century.

More recently, the practice has been to cremate and secretly scatter the ashes of notorious criminals in some anonymous place. Cremation and secret scattering of the ashes also has the additional effect of removing all possibility of there being a grave to visit in the future. This was the fate of Nazi war criminals such as Adolf Eichmann, Hermann Göring, Heinrich Himmler, Fritz Sauckel, and Julius Streicher. The remains of British serial killers Myra Hindley, Dr Harold Shipman, and Fred West were treated in the same way. A similar proceeding was carried out with the remains of Martin Bormann, who committed suicide shortly after the fall of Berlin in 1945, and whose remains, found in 1972 and positively identified by 1998, were disposed of in the Baltic Sea in 1999.

After he was killed in 2011 as part of Operation Neptune Spear, the body of Osama bin Laden was placed in a weighted plastic bag and made to sink into the sea at an undisclosed location. A year after his death, the headstone of disgraced television presenter and alleged sex offender Jimmy Savile was removed and destroyed in 2012, three weeks after being erected, when posthumous allegations of sexual abuse over decades came to light. The ashes of Abimael Guzmán, the leader of the Peruvian Maoist terrorist organization Shining Path, who died in prison in 2021, had his remains secretly disposed of by Peruvian authorities. In 2022, during the Salvadoran gang crackdown, graves of Mara Salvatrucha members in El Salvador were ordered to be destroyed, and prisoners were sent to smash up tombstones and remove gang-related graffiti.

In Judaism, contact with a corpse confers uncleanness (see Numbers 19:11-22 and Tractate Oholoth in the Mishna). Cohanim, descendants of Aaron, are prohibited from approaching within 4 cubits of a grave, except for when a funeral is of a close relative. Thus, an unmarked grave opens up the possibility that a pious Jew could become defiled without being aware that it happened. The Jews of early times, therefore, sought to avoid unmarked graves by two means: clearly designating cemeteries beyond the limits of their villages and cities, and making graves and tombs obvious by whitewashing them. This is the background for Jesus's comparison of the Pharisees of his time to white-washed tombs (see Matthew 23:27-28) and to "unmarked graves, which men walk over without knowing it" (Luke 11:44). Jesus warned that the Pharisees were defiling others by their hypocrisy, misplaced priorities, and selfish ambition.^{[citation needed]}