Marn Grook, marn-grook or marngrook (also spelt Marn Gook) is the popular collective name for traditional Indigenous Australian football games played at gatherings and celebrations by sometimes more than 100 players. From the Woiwurung language of the Kulin people, it means "ball" and "game".

These games featured punt kicking and catching a stuffed ball. They involved large numbers of players, and were played over an extremely large area. The game was subject to strict behavioural protocols: for instance all players had to be matched for size, gender and skin group relationship. However, to outside observers the game appeared to lack a team objective, having no real rules or scoring system. A winner could only be declared if one of the sides agreed that the other side had played better. Individual players who consistently exhibited outstanding skills, such as kicking or leaping higher than others to catch the ball, were often praised, but proficiency in the sport gave them no tribal influence.

The earliest accounts emerged decades after the European settlement of Australia, mostly from the colonial Victorian explorers and settlers. Historical reports support it as a widespread activity across south-eastern Australia of the Djabwurrung and Jardwadjali people and other tribes in the Wimmera, Mallee and Millewa regions of western Victoria. According to some accounts, the range extended to the Wurundjeri in the Yarra Valley, the Gunai people of Gippsland, and the Riverina in south-western New South Wales. The Warlpiri people of Central Australia played a very similar kicking and catching game with a possum skin ball, and the game was known as pultja. North of Brisbane in Queensland in the 1860s it was known as Purru Purru.

Some historians claim that Marn Grook had a role in the formation of Australian rules football, which originated in Melbourne in 1858 and was codified the following year by members of the Melbourne Football Club. This connection has become culturally important to many Indigenous Australians, including celebrities and professional footballers from communities in which Australian rules football is highly popular.

Although the consensus among historians is that Marn Grook existed before European arrival, it is not clear how long the game had been played in Victoria or elsewhere on the Australian continent.

A news article published in 1906 suggests that it had been observed around a century prior, which would put its first observations to Australia's earliest days as a convict colony.

The earliest recorded anecdotal account is from about 1841, a decade prior to the Victorian gold rush. Robert Brough Smyth in his 1878 book, The Aborigines of Victoria, quoted William Thomas, a Protector of Aborigines in Victoria, who stated that he had witnessed Wurundjeri Aboriginal people east of Melbourne playing the game:

The men and boys joyfully assemble when this game is to be played. One makes a ball of possum skin, somewhat elastic, but firm and strong. ...The players of this game do not throw the ball as a white man might do, but drop it and at the same time kicks it with his foot, using the instep for that purpose. ...The tallest men have the best chances in this game. ...Some of them will leap as high as five feet from the ground to catch the ball. The person who secures the ball kicks it. ...This continues for hours and the natives never seem to tire of the exercise.