The German Nature and Biodiversity Conservation Union (NABU) awarded Großmann the title on Wednesday because of his push for an extension to the use of nuclear power in Germany and his commitment to coal-fired power.
Großmann took the criticism in his stride.
“I'm very grateful,” he wrote in a message to NABU, and asked when and where he could collect his award.
NABU has offered the “dinosaur of the year” award since 1993 to people who, in its view, stand out for “particularly enduring stupidity” on environmental and climate policy issues.
“With unscrupulous and provocative lobbying of the federal government for the extension of the lifespan of nuclear reactors, which culminated in the late summer with an advertising campaign initiated by him, Mr. Großmann has justly earned the prize this year,” NABU president Olaf Tschimpke said in a statement.
In September, the government approved an extension to the running times of Germany's 17 nuclear reactors by an average of 12 years. This led to widespread accusations of cronyism and back room deals with energy companies.
Tschimpke said share of renewable energy in RWE's mix was just 3 percent, a large part of which was supplied by ageing hydro-electric plants.
The firm, one of Germany's big four energy companies, stands to make an enormous profit from the nuclear extension, the NABU statement said.
Germany's Institute for Applied Ecology, a private research institute with connections to the anti-nuclear movement, would reap additional profits of €17 billion from the extension.