The administrative court in Paris ruled that the government's failure to convert its commitments on reducing greenhouse gas emissions into policy made it “responsible… for some of the ecological damage seen”.
The judges said however they would take a further two months to decide whether it was necessary to force President Emmanuel Macron's government to take steps to ensure it met its targets on greening the economy.
Oxfam France in a Tweet hailed the ruling as a “historic victory for the climate”.
“The state's climate action has been recognised as ILLEGAL,” it added.
Oxfam France, Greenpeace France and two other groups accused the government of failing to keep Macron's stated promise to “make our planet great again”.
The case is part of a mounting push by climate campaigners across the world to use courts to pressure governments into action.
An international accord signed in Paris five years ago aims to limit global warming to less than 2C (3.6 Fahrenheit) over pre-industrial levels, and preferably to 1.5C.
But experts say governments are far from meeting their commitments and anger is growing among younger generations over inaction, symbolised by the campaigns of Swedish teenager Greta Thunberg.
In a separate but similar case in November, France's top administrative court gave the government a three-month deadline to show it was taking action on global warming.
That case was brought by Grande-Synthe, a low-lying northern coastal town that is particularly exposed to the effects of climate change.