Mario Party 9 was announced at E3 in which year?

Mario Party 9 was announced at E3 in 2011.

Nintendo revealed the ninth main Mario Party game at the 2011 Electronic Entertainment Expo. The announcement preceded its release in 2012, making 2011 the announcement year rather than the launch year.

Mario Party 9 was developed by NDcube for the Wii and introduced a major structural change: players moved together in a vehicle instead of independently traversing a board. The game also replaced the traditional Star objective with Mini Stars collected during each stage.

That timeline causes the most common confusion. Mario Party 9 is generally identified as a 2012 video game because its releases began in Europe, North America, and Australia in March 2012, followed by Japan the next month. Its E3 announcement, however, took place in 2011.