Local media reported smoke could be seen coming from the volcano and Iceland’s Meteorological Office confirmed that an eruption had begun.
Grimsvotn lies under the uninhabited Vatnajokull glacier in south-east Iceland.
It last erupted in 2004. Scientists have been expecting a new eruption and have said previously that this volcano’s eruption will likely be small and should not lead to the air travel chaos caused in April 2010 by ash from the Eyjafjallajokul volcano.
Sparsely populated Iceland is one of the world’s most volcanically active countries and eruptions are frequent.
They often cause local flooding from melting glacier ice, but rarely cause deaths.
Last year’s Eyjafjallajokul eruption left millions of air travellers stranded after winds pushed the ash cloud toward some of the world’s busiest airspace and led most northern European countries to ground all planes for five days.
In November, melted glacial ice began pouring from Grimsvotn, signalling a possible eruption. That was a false alarm but scientists have been monitoring the volcano closely ever since.