The Mariu were an indigenous Australian people of the Northern Territory. Their language is unattested, but may have been Miriwung.
In Norman Tindale's estimation the Mariu's territory covered some 1800mi2 in the area south of where the Victoria River enters the Joseph Bonaparte Gulf. The Bullo River also formed part of their land.
In 1900, R. H. Mathews, a surveyor and self-taught anthropologist, presented a paper attributing to the Mariu — together with other "large and important tribes", from both Western Australia and the Northern Territory, such as the Gija, Perrakee, Gooniyandi, Nyigina, Bunuba, Djaru and Walmadjari — an 8 section marriage system among clans, which he illustrated in the following general table.
| Phratry | Father | Mother | Son | Daughter |
|---|---|---|---|---|
| A | Jungurra | Nungulla | Jabulgie | Nabijerry |
| Jackara | Nabijerry | Julimar | Naboron | |
| Janima | Naboron | Jungary | Nabungarty | |
| Jambidgena | Nabungarty | Jangula | Nungulla | |
| B | Jangula | Nangilee | Jambidgena | Nambidgena |
| Jungary | Nambidgena | Janima | Nabina | |
| Julimar | Nabanna | Jackara | Nackara | |
| Jabulgie | Nackara | Jungurra | Nangilee | |