President Obama made his remarks to students at Connecticut’s Coast Guard Academy.
President Obama has called climate change “a serious threat to global security” and heavily criticised climate change deniers.
In his strongest remarks on the issue yet, Mr Obama said climate change would “impact every country on the planet”.
The US president made his comments in a speech to students at the Coast Guard Academy in Connecticut.
He has previously identified climate change as a central issue for the remainder of his time in office.
Faced with strong opposition on the issue from Republicans and industry, Mr Obama used his speech to frame the threat in terms of national security.
He said: “I’m here today to say that climate change constitutes a serious threat to global security, an immediate risk to our national security and make no mistake: it will impact how our military defends our country.
“Denying it or refusing to deal with it endangers our national security. It undermines the readiness of our forces.”