Mario Party 9 is a party video game developed by NDcube and published by Nintendo for the Wii. The ninth main installment in the Mario Party series, it was announced at E3 2011 and released in Europe, North America, and Australia in March 2012, followed by Japan a month later. It was the first game in the series to be developed by NDcube, who took over development of the series from Hudson Soft, which was absorbed by Konami in March 2012. This was the last Mario game to be released in standard definition.

Mario Party 9 was the second and last game in the series released for the Wii, and was followed by Mario Party: Island Tour for the Nintendo 3DS in 2013 and Mario Party 10 for the Wii U in 2015.