Mourners are gathering in Iceland to commemorate the loss of Okjokull, which has died at the age of about 700.
The glacier was officially declared dead in 2014 when it was no longer thick enough to move.
What once was glacier has been reduced to a small patch of ice atop a volcano.
Prime Minister Katrin Jakobsdottir, environment minister Gudmundur Ingi Gudbrandsson and former Irish president Mary Robinson will all take part in the ceremony.
After opening remarks by Ms Jakobsdottir, mourners will walk up the volcano northeast of the capital Reykjavik to lay a plaque which carries a letter to the future.
"Ok is the first Icelandic glacier to lose its status as glacier," it reads.
"In the next 200 years all our main glaciers are expected to follow the same path. This monument is to acknowledge that we know what is happening and what needs to be done.
"Only you know if we did it."
The dedication, written by Icelandic author Andri Snaer Magnason, ends with the date of the ceremony and the concentration of carbon dioxide in the air globally - 415 parts per million.
"You think in a different time scale when you're writing in copper rather than in paper," Mr Magnason told the BBC. "You start to think that someone actually is coming there in 300 years reading it.
"This is a big symbolic moment," he said. "Climate change doesn't have a beginning or end and I think the philosophy behind this plaque is to place this warning sign to remind ourselves that historical events are happening, and we should not normalise them. We should put our feet down and say, okay, this is gone, this is significant."
Oddur Sigurdsson is the glaciologist at the Icelandic Meteorological Office who pronounced Okjokull's death in 2014.
He has been taking photographs of the country's glaciers for the past 50 years, and noticed in 2003 that snow was melting before it could accumulate on Okjokull.
"Eventually I thought it was so low that I wanted to go up there and check myself. I did that in 2014," he said. "The glacier was not moving - it was not thick enough to stay alive. We call that dead ice."
The glaciologist explains that when enough ice builds up, the pressure forces the whole mass to move.
"That's where the limit is between a glacier and not a glacier," he says. "It needs to be 40 to 50 metres thick to reach that pressure limit."
Anthropologists Cymene Howe and Dominic Boyer from Rice University in Texas made a documentary about the loss of the glacier called Not Ok in 2018, and came up with the idea of a memorial during filming.
"It's not the first glacier in the world to melt - there have been many others, certainly many smaller glacial masses - but now that glaciers the size of Ok are beginning to disappear, it won't be long before the big glaciers, the ones whose names are well recognised, will come under threat," Dr Boyer said.
Glaciers have great cultural significance in Iceland and beyond. Snaefellsjokull, a glacier-capped volcano in the west of the country, is where characters in Jules Verne's science fiction novel Journey to the Centre of the Earth found a passage to the core of the planet. That glacier is now also receding.
Mr Sigurdsson made an inventory of Icelandic glaciers in the year 2000, finding there were just over 300 scattered across the island. By 2017, 56 of the smaller glaciers had disappeared.
"The oldest Icelandic glaciers contain the entire history of the Icelandic nation," he said. "We need to retrieve that history before they disappear."