SumOfUs says the deal "would radically threaten global food systems and undermine food security for millions of people".
The petition asks Werner Baumann, chief executive officer of Bayer, to kill the merger. The petition will also be delivered to regulatory authorities in the United States and Europe "highlighting the risk that a Monsanto/Bayer agrochemical monopoly would create for food systems, consumers, and the environment".
According to SumOfUs, the proposed merger would mean a company with a $67 billion turnover that would control most of the US corn, soybean and cotton market. SumOfUs also expressed fears about neonicotinoids and pollinators and about glyphosate.
Monsanto and Bayer have been asked for comment.