The airline is among 30 firms targeted by Milieudefensie.
That's the Dutch wing of Friends of the Earth, and the group behind a big legal win over Shell.
Back in May a court in the Netherlands ruled that the oil giant must move faster to cut its emissions.
Now the heads of 30 new firms have received letters from the activists.
The letters demand that the companies set out plans to trim their emissions by 45% from 2019 levels, in line with the Paris climate accord.
Failure to do so is likely to trigger legal action.
It's all part of growing global move to tackle climate change in the courts.
Research by the London School of Economics shows that more than 1,000 climate-related cases have been brought since 2015.
Besides KLM, other firms on the new list include supermarket chain Ahold Delhaize, and ABN Amro bank.
The lender said it was committed to fighting global warming, and was working to reduce emissions connected to its business and customers.
As for Shell, it's appealing against the court order requiring it to cut emissions in line with the Paris agreement.