The player can obtain the Venusaurite by completing Mission Card 15.
The Venusaurite was also available as a prize during the Mega Venusaur Competitive Stage. This Competitive Stage ran on the Nintendo 3DS version from May 29 to June 8, 2015 and August 24 to 31, 2015. It ran on the Pokémon Shuffle Mobile from February 23 to March 1, 2016.
The Venusaurite enables the following Mega Evolutions:
A Venusaurite belonging to a Trainer appeared in Mega Evolution Special IV, during the ten-battle gauntlet Lysandre had arranged for Alain's training. The Venusaurite is located on a band placed around the flower on Venusaur's back.