Images from a Nasa satellite show the island breaking off a tongue of ice that extends at the end of the glacier.
In 2010 an ice island measuring 250 square km (100 square miles) broke off the same glacier.
The process that spawns icebergs - known as calving - is a natural, periodic process affecting all glaciers that terminate at the ocean.
A previous calving event at the same glacier in 2010 created an iceberg twice the size of this one.
Scientists have raised concerns in recent years about the Greenland ice shelf, saying that it is thinning extensively amid warm temperatures.
No single event of this type can be ascribed to changes in the climate.
But some experts say they are surprised by the extent of the changes to the Petermann Glacier in recent years.
"It is not a collapse but it is certainly a significant event," Eric Rignot from Nasa said in a statement.
Some other observers have gone further. "It's dramatic. It's disturbing," University of Delaware's Andreas Muenchow told the Associated Press.
"We have data for 150 years and we see changes that we have not seen before," Mr Muenchow added.
The calving is not expected have an impact on sea levels as the ice was already floating.
Icebergs from the Petermann Glacier sometimes reach the waters off Newfoundland in Canada, posing a danger to shipping and navigation, according to the Canadian Ice Service.