Officials, activists and others made the two-hour hike up a volcano to say goodbye to what was once a glacier.
Previously named Okjökull, it is now just nicknamed OK, minus the Icelandic word for glacier.
Children installed a memorial plaque at the spot of the former glacier, which read: "A letter to the future.
"In the next 200 years all our glaciers are expected to follow the same path.
"This moment is to acknowledge that we know what is happening and what needs to be done. Only you know if we did it.
Icelandic geologist Oddur Sigurdsson pronounced the Okjokull glacier extinct about a decade ago, but on Sunday he took a death certificate to the made-for-media memorial.
The moment was marked with poetry, silence and political speeches about the urgent need to fight climate change.
This was the first of Iceland’s glaciers to disappear, but Mr Sigurdsson said all of the nation’s ice masses would be gone in 200 years.
Icelandic Prime Minister Katrín Jakobsdóttir called the glacial loss a consequence of the climate crisis.
She said: "This is the first Icelandic glacier that's formally declared an 'ex-glacier'.
"But if the predictions of the scientists... if we see them happening, we will see other glaciers disappear in the next decades and centuries, which is obviously a very big thing for our landscape, nature, ecosystem, but also for our energy system because we produce renewable energies from the glacier rivers.
"I think it's so important for every leader of the world to be conscious of that. We are seeing the faces of climate crisis differently around the world, but it's the same crisis.
"And we need international cooperation on an unprecedented scale. We really need to rethink the way we are working to meet this challenge."